I actually managed to complete this. I get that a game shouldn't be too easy but a lot of parts in the game felt unfair. This kind of difficulty is more fitting for a game that creates a checkpoint every room, but this game used checkpoints very sparingly. The character is floaty in the air but that is a design choice, not a flaw. However, after getting the glider upgrade a little extra control in the air would be nice.
A big flaw was the wall jumping. The vertical boost when wall jumping should be way higher and the fact that you had to keep using the directional buttons towards the wall when jumping felt a bit annoying.
The final boss was nice but I don't get why I had to do the first part 2 times, that was just weird.
The general theme of the game was great. When in the last part of the game the background and skulls really made it feel like a gameboy game. The music was also fitting.