The controls are bad. The jumps have too much pull-down to them. The platforms are too picky and you'll slide off a lot. And if you manage to get somewhat far, one fall and you have to do it all over.
Okay, how the jumps should work (but don't) -- When you have multiple jumps and you use them, he jumps to the maximum combined height of all the jumps. Instead, you have to approximate the perfect time for each next jump, and it's pixel-perfect or very near it. What I mean by there's too much pull-down -- he falls too fast after a jump.
Combine that with all those slopes that throw you off them, as well as the game not having flawless latency, and you're in for a bad time. Would have been better without slopes throwing you off them, honestly, considering how picky the game otherwise is.