I decided to replay this game after playing through all of the four main classic Sonic games, just for fun. I expected a really bad experience, but... I kinda liked it?
I think the main reason people complain about this game is because of its title, and I think it is legit. If you play a game called "Sonic 4" you naturally expect something that feels like the classics gameplay-wise, so I understand the criticisms on the game engine and the weird physics, but personally I never found any situation in the game that demands momentum-based action. If you see the levels as some automated, almost endless runner-like obstacle courses (and I think it makes sense, since this was originally planned to be a mobile game, before being rebranded for nostalgia's sake) maybe you would enjoy this game a little more, but don't get your expectations too high, of course.
I think the main reason people complain about this game is because of its title, and I think it is legit. If you play a game called "Sonic 4" you naturally expect something that feels like the classics gameplay-wise, so I understand the criticisms on the game engine and the weird physics, but personally I never found any situation in the game that demands momentum-based action. If you see the levels as some automated, almost endless runner-like obstacle courses (and I think it makes sense, since this was originally planned to be a mobile game, before being rebranded for nostalgia's sake) maybe you would enjoy this game a little more, but don't get your expectations too high, of course.