Ace has been underrated despite all the facts. People for years didn't believe he had Haki despite the fact he'd been a New World Pirate for years which was unreasonable. Happy to believer every other commander had it, but Ace & Blackbeard didn't which is ridiculous.
Ace is beyond the strength of a regular Vice Admiral at the very least. In his first year, he defeated a VA, something not even Luffy could do. Given the fact that Ace, Sabo and Luffy have extreme grow rates, it's only reasonable to assume he was high ranked.
The point is he was the only one besides Blackbeard who was seen as real threats in regards to becoming Pirate King bar the Emperors. That alone automatically indicates that he's outstanding.
You can't be said to not have shown any potential without exhibiting strength at the very least, if Sengoku, and Chinjao rated them so highly. Sengoku and co. never said the same regarding Luffy despite his feats. or any other pirates despite how strong they were. It's not an acknowledgement easily gained if Blackbeard is only gaining that for taking out Ace and then Marco and co. to become an Emperor.
If you also read what Chinjao said, he based Blackbeard's prospects as the future PK
based on he strength he displayed. Watch what he said regarding Luffy? He said if you can only "
match" me in my current state, then you don't have potential to be PK. Only makes sense that Ace proved himself via his strength in order to be assessed as a potential because he was powerful. There are many other powerful Pirates, but only Ace & Blackbeard stood out. Not 10 different potentials.
The fact that he had a Bounty value of 550m further proves exactly what I stated earlier. 3 years to accumulate that is quite the feat.
As Blackbeard also said, it's been a while since you've been hit. Being a Logia doesn't guarantee not being hit especially by Haki users and more so as he's stationed in the New World. Could very well imply he had top-notch abilities in evading damage like Katakuri. We really do not know. Nonetheless, to defeat a VA before even getting to the New World, and given the growth rate he had especially as he was a WB Pirate and Pirate for an entirety of 2 years, it's very likely his strength is underestimated. There might not be many acknowledged feats. but the people he tangoed with didn't have easy times. He's definitely underestimated.