Just got 100% in the main story mode, planning to try the bad guys story at a later point. The end-game levels are definitely very good, I agree! The only thing I'd really criticize is that the "snaptrap" enemies in one of the chase sequences that's super zoomed-out are really hard to see since they're small and blend in really well with the color palette of the level. They were easy enough to avoid once I knew they were there, but I ran into them a handful of times without seeing them.

I did run into one particular glitch during my playthrough; I managed to clip right through the rotating "graham cracker" platform twice. I think I got into wall-jump position right before it started acting as a floor, so I ended up falling right through the platform entirely. Not sure if you're aware of this already!

I was going to suggest that the homing fireball projectile (the thing that gave me the most trouble for sure) should be an unlockable character as a joke, but then I got 100%...