The fanbase bitches because of stupid reasons like story, characters or voice acting.
...the reason the games don't get any respect is because... well, they're bad. They're playable by people who force themselves to go through the awful gameplay or stage design, but I don't give a [tornado fang] how many conspiracies people dream up for hatred on Sonic games. The games are pure and simply not up to standards with any other decent-quality games in today's gaming world. Lots of games, like the Sonic Adventure titles, still hold up quite well, and yes, the Advance and Rush titles were very good. But almost the entire rest leaves something to wish for on every single title that deems them "bad".
Problem is that neither fans nor the company can make up their minds. But right now, they seem to be going on a good direction. Sega previously just ran away from every single gameplay combination they couldn't make work. They seem to be attempting to make Unleashed's gameplay finally WORK according to the damn levels, with design that fits the gameplay, colorful and playful settings, and an actual good quality game.