We have absolutely no reason to doubt August or Zeref though, and Laxus being able to casually one-shot Ajeel really isn't surprising when you consider he outright
killed Wahl the moment he bypassed his immunity.
I'm guessing the three Spriggans you're talking about are Lessio, Neinhart and DiMaria. Let's have a look at these scenarios:
Lessio: He had a prolonged fight with him in base and had to go FDKM to one-shot him - valid
(Enhanced) Neinhart: As you said, he had POF here - valid but suspect
DiMaria - Partial E.N.D did this. Natsu can't access that power at will - invalid
So he beat two Spriggans, but one of them has one of the worst durability in the series + was POF'd and the other took multiple named spells from base Natsu + Lucy and still required an FDKM Demo Fist.
Compare this to Laxus
killing Wahl the moment his immunity wasn't a factor and being hyped as able to casually kill/severely injure Ajeel. Natsu's feats don't stack up that well in comparison.
He's been useless because he's hardly had any panel time. Laxus was pretty much kept out of the story to stop him from stomping any Spriggan (not named August, Irene or Larcade) he ran into. The same could be argued for Gildarts and Jellal.
While true that Laxus needed an asspull/POF to beat Wahl, that was only because Wahl was a direct counter to Laxus. It was the equivalent of putting Natsu up against God Serena.
Also LMFAO at suggesting Laxus is inconsistent. Amazingly enough, Laxus has been among the most consistent characters of the arc, both in hype and feats. At the start of the arc he was basically hyped to one-shot/severely injure Spriggans if he casually nuked them, despite being infested with MBP's. Keep in mind, these words come from the magic king, so there's no reason to doubt them, especially since he wouldn't gain anything from lying about this. He then goes on to fight Wahl in Hargeon, one-shotting and
killing him the moment he bypasses his immunity and is free of MBP's. This is pretty damn consistent and makes the Ajeel hype look pretty plausible, since the difference between KO'ing and killing is massive, and the difference in performance against Ajeel and Wahl was caused by the MBP's. After this he fights Historia Hades and wins, despite being half-dead. All of this shows off his power, endurance and, more to the point, consistency.
Gildarts could easily crash Deus Sema, his entire magic is built around destroying objects. And as for Laxus, a few bolts of lightning should destroy Deus Sema. At the end of the day, the dangerous part of Sema/Deus Sema is the blast wave, not the meteor itself, since (as Erza showed) you can destroy it before it lands. And while yes Erza did have POF, a serious Gildarts/Laxus should be far more powerful than a crippled Erza with POF.
It's true that August was pretty lax against Gildarts, but the big difference is about the nature of August's magic in comparison to the nature of human Irene's enchantments. Whilst enchantments can be annoying, they can be dodged and don't stop an enemy from damaging their target. On the other hand, August's copying/nullifying magic basically turns the fight into a stomp as the enemy's magic is useless whilst August can use his spells and is a beast in CQC.
What I'm trying to say is that while giving human Irene some trouble is impressive, it's not easy to compare it to not getting stomped by August, as human Irene and August are vastly different types of foes. Human Irene is simply strong, August is stronger, immune to anything you throw at him that isn't holder magic, has incredible CQC skills and has immense durability.