Three problems
1: The hitboxing was awful, just like in 90% of flash games on NG. I don't understand why people have such a hard time drawing a rectangle equal to the size of an object.
2: The controls were SEVERELY delayed. You had to jump well before it appeared so or you'd double jump off a platform, for example.
3: The whole "floating forward" thing that happens when you jump isn't a huge deal, but is annoying.
Aside from those it wasn't too bad. I was expecting a lot of pixel-perfect jumps and stupid shit like that, but blissfully they were nowhere to be found. Challenging but beatable.