Sprite quality and animations are superbe. The background in the first chapter would look amazing with some paralax. Shooting is really satisfying.
A lot of the enemies fire their projectiles too high for you to shoot down, but too low for you to duck under. In combination with the high projectile speed, and their ability to shoot you off screen, I feel like enemies are bit more dangerous than they need to be.
I feel like the platforms in the earlier jungle sequence could be more evenly spaced to help learn the timings / distance on jumps better. Theres also a leap of faith one where you have to jump onto a small platform, but doing so moves the platform out of your field of fiew, and with no other objects on screen at that point, it makes it really difficult to gage whether you're going to overshoot or not.