Was it your intention to have the jumps be hard as balls to control? Since before the days of the NES, letting go of directional input during a jump has meant "slow your horizontal motion." Here, you retain all of your horizontal momentum, and it is the source of a great, great many difficulties.
Also on the list of things that are incredibly annoying is the complete lack of ability to change your direction in ball form. If you were intending the player to learn to push down ONLY and PRECISELY when striking the floor, this is fine (though you should note this somewhere in the game so the player doesn't feel deceived). Otherwise, a little bit of control during at least before the first bounce shouldn't be THAT bad, right?
On a more micro level, I think the coins may be slightly glitched; at least in level 7, where I passed through the coin and it didn't recognize the collection until I jumped at it from below. Looks like maybe the "hitbox" for it is lower than the actual sprite. Slight issue, that.
Also at one point I encountered an area where I was falling down a crevasse maybe ten blocks deep, and momentarily glitched in such a fashion that I ended up standing inside the wall, having landed there from the fall. I wasn't able to replicate it (though admittedly I did not try very hard) but it might be something to look into.