What made them the strongest humans is their lunarians genes or the genes from YCs level like Jimbei.
Did Luffy complain about their abilities or their lunarian hax?
Fraudkuri's gimmick was his Future Sight in which, if it gets bypassed he became ass and fodder to Gear 4th and if he lost his calmness he gets ragdolled.
What made them the strongest is the fact how Vegapunk created the strongest humans with various mixed genes. I'd say their Lunarian genes are the most dominant ones.
Nothing bad about that honestly and like I said, they have some sort of battle IQ because they refused to ever use their speed mode when fighting against Yonko Luffy, below-Lord Kuri level Lucci, Zoro and Kaku, unlike King who was even switching between his modes against ACoC Yeti Cool Brother's victim whom - since his fans swore to it - reached Prime Oden level AP at this point. That's a very poor example of low battle IQ in which you have to question if King is actually dumb or not, right?
Why is everyone a fraud according to you??? Kuzan is a fraud, Akainu is a fraud, Garp is a fraud, Aramaki is a fraud for making a black blade as a side quest, Shanks is a fraud - did I miss anyone else? So how is he a fraud for having an advanced Haki as a "gimmick"? He's at the strongest with his CoO, so how is that a gimmick? I don't follow you.
Fighters get overwhelmed if they lose their composure - this was a thing ever since Water 7. And this was Zoro against Lucci without his own composure:
(Don't mind the meme, I needed a quick search to prove my point)