Having to boil down any lapse in logic down to plot. Yes, I get certain characters can't die or that Oda wants characters in a certain location, but it's the author's job to make it believable. And I feel One Piece fails at believably getting our characters out of a jam or putting them in place for a plotpoint. It's a lot worse post timeskip, when Oda insist on making things harder for the protagonists, yet none of them suffer any real repercussions, or their escapes are far too convenient at times.
The most egregious example of plot convenience would be the Birdcage, which was just an excuse to prevent any of the other able bodied combatants from helping Luffy against Doflamingo, so that the fight could be more dramatic. Or how recently in Wano, Oda has characters teleport all over the map at inconsistent intervals just to set them in place for the next plot point. It's artificial and lazy story telling.