Some players who joined my co-op game helped me uncover a couple of bugs which will hopefully be fixed in the next version. Those are:
Twirl attack can somehow get disabled during multiplayer special stages (intermittent, cause unknown)
Holding the Spin button while in goop rapidly plays the "slam" sound
Potential softlock in CEZ3 (believed to be related to using the slam attack)
The audio loop while gliding underwater isn't seamless, though it's quiet so you usually can't hear it
The battle against Metal Sonic (at least the last hit) took a long time even with four players, so the difficulty in singleplayer might be unreasonable. That's the only part I couldn't get past, but I put it down to a lack of skill; additional feedback will be appreciated.
I hope there aren't any I'm forgetting. If you find any issues other than those listed, please let me know!