Hard to see stuff
Eh my biggest complaint is how hard it is to see what's actually going to hit you.
The screen is awfully cluttered with effects and is pretty dark, and being able to move into the background makes it much harder to see what stuff is actually in the same row.
Perhaps just having 3 fixed depths and moving between them would be easier than having to fix your depth precisely.
Also the bosses have a bit too much HP to the point of getting boring, same goes for repeating the same obstacles too many times.
Music also get's repetitive.
However I liked the graphics, especially the 3D stuff, and I'm sure I'd like the game if I had more time to practice.