The only real legitimate complaint is ACoC, Conqueror infusion. This is the worst ultimate power-up ever, one that is wanked to high heavens by Oda and the cast but there is no good cue of when it’s being used or why characters do not use it and do not comment on it.
We don’t argue when is CoA or CoC being used cuz the characters are assumed to be using it to a certain extent. But Goofy or Zoro not using ACoC in battles where they can’t stomp, Yonko like BM not using it in battles where she goes all out. This shit is annoying
The rest is kind of nitpick and can be trivialized.
My main hangup with Haki, most of all really, is just how vague even the most basic of applications feel, not everything has to be explained and I can get that. CoO sort of gets a pass in that regard.
Other times you can hit another person while both using CoA, but then 'oh their Haki is stronger so you feel the pain while they bludgeon your face in'.
Before hand, there was very little in One Piece involving powers that were just straight RPG-style buffs to your "power". Typically upgrades had benefits beyond "you hit harder" so they were more diverse. The only thing I can think of that super closely matches is Asura and Zoan Fruits in general, but Zoan Fruits kind of make sense due to the nature of gaining animalistic strength in the world of One Piece (which already ups the maximum strength a normal person can have, so it sort of compounds on one another).
IDK shit about Asura because that still hasn't been explained
Examples of amps or boosts that weren't PURELY for power that also don't have harsh costs are things like Gear Second, Diable Jambe, etc. Those are sick as fuck, hell even Gear Second still has shortcomings in how much of a stamina drain it is. Plus the harsh cost-amps are also fantastic, impact dials being so devestating, Gear Third making Luffy into the newest character on Dragon Ball Daima, even Wapol fucking eating some people
I feel like Oda was being really creative in the earlier arcs in how things were set up and balanced and as things go on, while I'm still obviously a huge fan, it's a bit lacking in some aspects of the fights. I feel like that's not TOO nitpicky.