There's been a lot of division lately among that fandom as well. I think it's a natural part of any series that's been around for a long time and has passed its prime, but one difference is that I can easily understand the complaints that One Piece fans have about that series more than I can understand some of the criticisms that Pokemon fans have about the Pokemon anime, because Pokemon has always been one of the more inconsistent anime and I figure that by now most Pokemon fans should be used to things not going their way.
One Piece and Pokemon have similar problems. There's a group of fans who like what it is, and another group who watch but like what they hope
it could be.
With OP, that subset of fans wanted it to be Dragon Ball Z 2.0, with an emphasis on martial arts and power levels and less silly adventure and comedy. The arc that first began the schism between the two groups I outlined was Skypia, but the big one was Enies Lobby which signified One Piece transitioning into a DBZ-clone.
In Arlong Park's private veteran forum, many of the long-time fans who liked OP since 2001 found that arc distasteful and left. But OP brought in all the deprived DBZ fans because it started copying what was more popular with them.
It's very telling that one of the loudest complaints against OP is Luffy's Devil Fruit. That's like complaining Goku is a Saiyan. The fans have become stupid beyond salvation. But it also highlights just how far OP's fanbase has mutated and responded to the direction the story went.
On the Pokemon side, there is a substantial number of fans, possibly the majority, who want the series continuity to mature instead of Ash doing the same thing every new region. Since Pokemon has
not, that voice hasn't changed quite as dramatically in 20 years. But there's also high turnover where most fans grow out of it quickly and the ones who stay do so because they want a satisfying resolution to the journey.