I have lots of matchup disappointments from finished arcs. Let's start with the most recent, and then go backwards in time…
-Zoro curb-stomping Monet was a wasted opportunity for one of the weaker characters to beat her in a truly epic match. Could the fight perhaps been moved outside of Biscuit room and to where Mocha keeled over via candy OD? That way, we'd trade in a weak Zoro sexism match in exchange for Chopper having a glorious toe-to-toe vs a formidable opponent, and some very justifiable haki awakening. Nobody'd expect that Chopper would be the fourth to awaken his haki, and it would've made for a much better narrative.
-Luffy VS Caesar so many damn times. Blatant filler and a waste of time. Lose the second CC fight, give us an extra chapter for Chopper VS Monet, and we're golden.
-Truth be told...Most of the damn arc. Nobody needs another reminder of how bad this arc was on a plot level (scenery, character design etc weren't), but several of the battles were especially bad. Although I like the "test" Hodi gave Luffy -- made a good first opponent.
-Any fight that made Sengoku look like a chump.
-Akainu being way too overpowered vs the WB Division commanders. He shouldn't be beating up on 12-13 guys all by himself, ESPECIALLY after getting bashed up by Whitebeard. IMO it really starved the significance of what Whtiebeard did, instead leaving his legacy as "oh he got a bunch of nasty-ass wounds" rather than "that dude took on the admirals and WON."
...And...eh. There are many other fights that frustrate me, but those are the only ones I have energy to moan about. It's mainly just Akainu having invicibility mode during Marineford that gets me the worst.