This game is cobbled together from old assets, has a rather thin plot heavily held up by good character writing and references to older Sonic titles, and has a lot of jank.
I love it.
It's really damn fun to play, and honestly the occasional jank is usually more fun than it is annoying (with the only real exception being the lack of enough invincibility frames). The character writing is really damn strong, using the limited overall plot as a way to explore characters that haven't been allowed to be characters in the games for years. The gameplay is really fun, even when it can be unrefined, and the level design, while varying from being a bit underbaked to "we just remade a whole level from Sonic Adventure 2", never manages to be actively unpleasant outside a few sections where the designers got a bit too aggressive with making players latch onto a 2d plane in the open world (mostly notably on Chaos Island). Also, every boss fight manages to be incredibly fun and over-the-top. Hell, even the basic ass fishing minigame which completely breaks the game's progression is fun, if only because you can catch some very funny things (Sonic will be dying from various neurotoxins from holding poisonous frogs). While undoubtedly flawed, the strengths of this game far outweigh the downsides, even if I would suggest waiting for a sale if you aren't sold on it. It's a solid basis for Sonic to build on in the future (and hopefully they won't somehow still manage to rush the next game forcing the more traditional levels to be filled with reused assets).