The criticisms I have of the game center mainly on the full campaign, even with the optional challenge gates, is really short. Like, six hours or less short. At $30, I don't feel too burned. At $60, I probably would.
There's also the fact that this game is pretty clearly rushed. The lead up to the last boss is simply...jumping past the center parts of gears. The last boss himself is an audio/visual/gameplay mess that basically encapsulates everything terrible about modern Sonic at once. Planet Wisp dragged on forever and one world per game was clearly a time-saving measure considering how much thematic overlap there is. This was a game that needed to launch next summer or next winter, but as the generational (hehe) transition starts, Sega likely wanted the game out now.
All that considered, the game is still really fun. Like, I enjoyed it a lot. The platforming has some baffling lows, but also so dizzying highs. As shitty as the Sonic Colors stage was (despite having literally no reason to be besides padding), some other stages hit the nostalgia buttons just right while marrying them to modern standards for fun and excitement.
Honestly, had this game been given the actual time it deserves and was properly finished, there would be no question that this would be Sonic's "Mario Galaxy" moment. As it stands, it's just a fun game most of the time.