>>32083104I just beat Pearl a month ago. I was really surprised with the amount of content it had, it's LOADS more than XY and Sun & Moon had. However, the story is really weirdly structured, and the climax relies on build-up from before you climb Mt. Coronet instead of having its own build-up at Spear Pillar. Also, the climax is SUPER anti-climatic, since nothing really happens at all despite the game telling you the universe is about to end. Nobody really reacts to it either. The cutscenes up there are also really ugly. And you have like zero interaction with your two rivals, at all, so they feel like complete strangers even after beating the game. Oh, and the battles are sooooooooo slow. Having just beaten FireRed right before doing Pearl, it felt like an eternity went by in every battle.
So I guess the reason for the Sinnoh hate is the slowness, badly paced story, the horrible climax and the rivals, which overshadows the great amount of content that the games actually provide. The games are objectively good, and there's not many things wrong with them, however the things that ARE wrong with them are just so impactful that the bad overshadows the good completely.