Just finished playing through all the levels! Yeah, I did find two small bugs:

I assume that the volume is supposed to increase or decrease in flat increments of 5%, but sometimes it offsets and increases by 4% or 6% instead. I'm assuming that might be some sort of rounding error?

This small tunnel in the Barrel Canyon level where the rolling spike balls filter out instantly kills you if you touch it. I assume the game thinks that I'm being crushed between the platforms.

Aside from those, I thought that most of the new levels and all of the bosses were solid! The game definitely has a steep difficulty curve, as increasingly difficult level designs combine with new, also increasingly difficult mechanics. I found myself starting to feel frustrated with the ice and the darkness mechanics in World 5, though. You seem to maintain the friction if you jump off the ice, and your jump ends up being locked into an arc that you can hardly adjust; it's completely different from how jumping off any other ground type works. This ended up really messing with me at multiple sections of the level and caused me to jump with nearly no horizontal speed and fall right into the adjacent pit a handful of times. I don't think the darkness is too bad, but I feel like the player could use a slightly larger viewing radius. I found myself being unable to see some of the hazards, especially the big spike ball enemies, until I had already moved right into them. I started feeling the frustration of being stuck with 0 hearts every time I respawned at the checkpoint at the final section of the level, and I ended up giving in and taking the 10 hearts freebie after dying a whole bunch of times.

The bosses don't seem to scale in difficulty nearly as much as the levels, especially since you always have two free hearts to grab and the fight checkpoints after you've done half of the boss's life bar. I felt like Larry was an extremely easy boss and not much tougher than Cinge from the first world. Francis was definitely an interesting fight, but I quickly figured out his pattern and didn't struggle with him at all after that. In my opinion, Mega Klaw is probably the most challenging boss of the bunch, but compared to the normal levels, they're all pretty easy.