Clans have their strengths and weaknesses. You used Fairies(magic oriented) and Giants(physical strength oriented) as examples, but Goddesses and Demons are nothing like that. I don't consider offensive techniques superior at all, I said many times that Ark is much better.
Goddesses and Demons aren't inherently superior in magic or strength. They are similar to Humans, it depends heavily on their own, natural talents as persons, unrelated to their racial traits. Ludo and Mael are clearly physically superior to Sariel and Tarmiel. Elizabeth is clearly more magic oriented while Nanashi is a physical fighter. Same for Demons, Meliodas is a physical fighter first, same for Galan, Zeldris, Derrierie. But there are other Demons like Monspiet, Mera, Gowther.
The difference between the goddesses and demons are in what their racial powers focus on. The goddesses have a versatile set of abilities with a big emphasis on support while the demons are almost exclusively built around offense, in particular with their demon mark buff which the goddesses lack an equivalent of. When you compare Tristan's unbuffed goddess side to his mark-buffed demon side in terms of offensive power you're getting an unfair comparison similar to if you compared a fairy and a giant in terms of physical feats. If you want a fair comparison you can just look at how Tristan's two magics interact when he's using both as basic elemental projectiles at the same time. They were equal. When I say you consider offensive techniques as superior I'm not referring to Ark vs PoD but just offense and defense period. Tristan being able to do more damage with his demon side is the argument you use to prove that this side is stronger, but you wouldn't use the same logic to argue that him being able to heal more effectively with his goddess side proves that is is stronger.
You say that goddesses and demons don't have the same distinct strengths and weaknesses and the fairies and giants but there's a clear difference in whether the two clans favor magic or physical strength. You're skewing your perception by focusing on the exceptions even though they exist for every clan. Fairies are physically weak but Dahlia is a brawler. Giants are physically strong but Dubs has little to no physical ability. The goddesses we've either seen in action or who have had their stats revealed skew heavily towards magic, think Elizabeth, Sariel, Tarmiel, Jenna and Zaneri. Nanashi is the only dedicated physical fighter in the clan, with Ludociel and Mael combining both magic and melee in their fighting styles, though their strongest attacks were purely magical. The demons in contrast only have a couple of dedicated magical fighters, those being Melascula, Monspeet and Gowther. Even Chandler who was the clan's mage was a physical powerhouse. The preferences of the two clans can be seen in their leaders as well. The DK is primarily a physical fighter while the SD is pure magic. That's why their abilities are what they are by the way. A magical fighter like the SD is heavily disadvantaged against The Ruler while the SD's intangibility to physical attacks is a problem for the DK. The demons and the goddesses are opposites in more ways than just their elements.
I don't see how Ark is a shield while PoD is a sword though? Ark can be used for offensive, defensive, healing purposes and is naturally superior to PoD. Same for PoD but the healing is far inferior and PoD can also increase negative feelings.
I brought Ludociel for that very reason. He is like what Tristan was trying to become. He is extremely powerful, relies on both CQC and magic but he also uses White Magic in a way Tristan can't. We saw him use Invigorate. Tristan can't even use such a technique so far so even if you see Ark as a "shield" he is lacking.
Tristan wanted to become strong enough to defeat anyone without relying on his PoD. But so far, it seems that his Goddess powers, while strong aren't enough to defeat the foes he has been against.
Tristan obviously will learn to use them both at the same time but nothing indicates that his Goddess powers are comparable to his Demon powers. As for Ark and PoD cancelling each other out(can't remember when it happened), it's simply a matter of output. Tristan's Goddess powers are in no way weak, his Falling Stars was noted to be extremely powerful and he destroyed Mera's cocoon with a single slash, but his power output with PoD is far stronger.
Ark is a shield because it focuses on defensive and supplementary techniques, not because it doesn't have any offensive uses. Likewise PoD can be used to repair the user's body, which doesn't actually heal the damage according to Meliodas, but the ability is mostly offensive so it acts as more of a sword. This distinction is meaningful because it makes Tristan's two sides balanced. If his goddess side was equal in offensive power to his demon side and also had way more versatility on top of that then his goddess side would be objectively superior and there wouldn't be any reason to ever use his demon side outside of the rare scenario where an enemy is resistant to Ark. If the demon side is about offense and the goddess side is about defense then both will always have a purpose and Tristan can also take advantage of both at the same time, which is what the whole nephilim thing is about.
You bring up Tristan being lacking in terms of his goddess side but the same is also true for his demon side, the two problems just manifest in different ways and in a manner that fits my analysis. Goddess magic is distinguished by its versatility so Tristan's flaw in that area manifests as an inability to perform complex techniques. Demon magic is distinguished by its raw offensive power so Tristan's flaw in that area manifests as an inability to control that power. Both sides of his abilities are still incomplete and this is by design. Tristan dislikes his demon side because he can't control it, but he can't simply ignore it and focus purely on his goddess side because that side has its own problems too. If he managed to solve the issue with his goddess side first then he wouldn't need to develop his demon side any further. This is why mastery and acceptance of his demon side has to happen before he can master his goddess side. Tristan couldn't become like Ludociel because that would have permanently locked him out of the other half of his abilities. Tristan's powers and his problems with them are the way they are for a reason.
You claim that nothing indicates that Tristan's goddess powers are comparable to his demon powers but we've already seen an example of this directly in the form of the Annihilation technique. When Tristan forms his two elements into basic projectiles the two cancel each other out. This wasn't a matter of Tristan carefully adjusting the outputs to be equal. Tristan himself didn't even know what he was doing and was merely following Gawain's instructions since she's the one who came up with the technique. The fact that Tristan was able to achieve the desired effect on the fly with zero practice is evidence that it didn't require anything beyond forming basic elemental projectiles. We know he's unable to use many goddess techniques and also unable to control his demon side so I don't buy the idea that he nailed the exact balance on his first try because he's just that skilled. The reason you believe that Tristan's demon side is superior is because you compare the two based on their offensive output even though that's the area his demon side is focused on over everything else, as I've explained before. With how massive of a boost Meliodas got from his demon mark it's no surprise that Tristan's demon mark would allow his demon side to do way more damage than his goddess side that lacks such a buff, but that's an apples to oranges comparison. Annihilation is the closest thing to an apples to apples comparison we've seen so far and it paints a very different picture.
As I said above, if both PoD and Ark are used at the exact same level of power, Ark will overpower PoD, because PoD is weak to Ark. I compared a few times here to Popp's Medoroa from Dragon Quest(if you haven't read or watched the series, skimming trhough Episode 46 of the remake should be enough to understand what I mean).
The elemental weakness between light and dark magic goes both ways. They are supposed to mirror one another so it wouldn't make sense for one to just be outright superior for some reason. This is also consistent with everything else we've been told. Chaos-Melascula's nephilim monster was said to be a fusion of a demon and a goddess and therefore resistant to both dark and light magic. This establishes that demons and goddesses are resistant to their own element, but also that goddesses are not resistant to darkness because otherwise the addition of the demon side would have been redundant. The SD explained in the movie that she is resistant to the Sacred Tree's magic because it is similar to her own. If powers that are similar to one's own are easy to resist then it logically follows that powers that are very different would be the most difficult to resist. That's why Ark is very effective on demons since light magic is the opposite of dark magic, but this also works the same way the other way around. The aforementioned SD tanked everything Ban, King, Diane and Gowther could throw at her without any trouble but ultimately died to Meliodas and Zeldris incinerating her with Hellblaze since demon power was her weakness. There are also some other examples of this relationship, like Sariel in chapter 275 remarking that the power of darkness counters the power of goddesses, and the way Estarossa used his darkness to stop Tarmiel's Grace from activating a little bit earlier. In a nutshell, PoD is weak to Ark but Ark is also weak to PoD. Neither one is inherently superior.
I am sure I understood your words completely, but my understanding of them does not mean agreeing with them, especially when they lack logic and contradict the facts.
It's amazing how you talk about the G-Clan as if they were just glass cannons with powerful magic and hax when we saw how Tarmiel was physically stronger than anyone in the Ten Commandments except maybe Zeldris and Mael and Ludociel who had physical strength that rivaled if not even surpassed the likes of Zeldris (the 3/4 strongest demon in 7ds).
And you can't use diversity to justify how Tristan with G powers can't do anything against opponents he could easily defeat if he used his demonic powers.
Tristan G-side can't beat even base Schwartz when his demon mode beat Melagalande easily
Just because you're a magical type fighter doesn't mean you can't win or put up a decent fight
King was a magical fighter but he didn't lose all his fights or be weaker than any stronger opponent who better than him physicaly.
And using the "annihilation" technique to claim that his powers are equal is just deception.
It was not mentioned that you need to put two equal forces of darkness and light to use annihilation and even if Tristan did he was holding back his demonic powers at the time so that he would not lash out and harm the others.
The reason I said you misunderstood something is because your counterarguments have very little to do with my argument. For example, you described my position as being the equivalent of saying that "Nerobasta isn't weaker than Derieri, it's just a matter of power diversity" but that's not what I'm actually saying. I'm talking about the way the abilities of the demons and goddesses, as well as the other clans for that matter, have different areas of focus so depending on which area you examine you can get a very different result. A goddess being able to out-heal a demon is not proof of the goddess being stronger overall and a giant being able to punch harder than a fairy is not proof of the giant being stronger overall. Note that these are general traits, it's not like every member of clan x is superior to every member of clan y or that every member of clan z is perfectly equal. That's not what this is about. Similarly you say that I "talk about goddesses being just glass cannons" but that's not a claim I've ever made. If anything my position is the opposite since I've described the goddesses as being more defense-oriented, i.e. more of a shield than a sword. And no, being defense-oriented is not the same as only having defense. As seen with the above examples, you're arguing against claims I never made and misrepresenting my positions, hence why I assumed you misunderstood me. You seemed to take that as an insult but I was giving you the benefit of the doubt, because it's much better to make an innocent mistake than to intentionally twist someone else's words. I don't need you to agree with me but I would like you to be fair and honest with me.
As for the rest of your post, much of the things you mention are already covered in my response to Demonspeed above. Tristan's demon mode has way more offensive power than his goddess mode because that's the thing his demon mode is focused on. This is no different from a hypothetical fairy/giant hybrid akin to Tristan being able to punch harder in his giant mode. The demon mark in particular gives Tristan a huge boost just like what happened with his father. It's no surprise that activating the mark would allow him to overwhelming enemies his goddess powers can't defeat, just like how it wouldn't be surprising for Tristan's goddess powers to heal injuries his demon powers can't. I find it strange that you consider examining the Annihilation technique to be deception when it's easily the most direct comparison of his powers we've seen so far. We're not comparing healing or physical strength or any other area where we know one side has an advantage over the other, it's just pure magical energy produced when Tristan was in his hybrid mode with one eye in goddess mode and the other one in demon mode. It's like comparing punching strength between a person's right arm and left arm. How is that not more accurate than measuring them in different situations doing different things? As for the way Annihilation works, Gawain says that it's the outcome of light and dark cancelling each other out and the name is taking from the physical phenomenon that involves opposite but equal particles cancelling each other out to release energy in much as same way as the "particles" Tristan formed out of light and darkness. It's pretty obvious what's going on there. You're wrong about Tristan holding back his demon powers so that he wouldn't lash out. The thing that causes Tristan to lose control is exceeding his time limit, which was established in the Liones arc and directly referenced before using Annihilation for the first time:
Tristan doesn't say anything about having to hold back his demon powers, just that he only has a few seconds left before he'll go crazy. If losing control was a matter of power output then it would be easy for him to control his demon mode just by limiting its power output. That way he could stay in that mode for extended periods of time instead of only doing it in short bursts.
