Disclaimer: I didn't actually beat the game!
So I really like this game, but I feel like the difficulty is handled wrong. Especially with the huge spike with the flying robo spike shooters. When you combine them with the goop ceiling shooter and the electric floor and a jump that is waaay too wide and a checkpoint that is way behind 3 other enemies, it all adds up to a huge difficulty spike that I just couldn't stomach. I sat and tried to beat that for like half an hour before giving up!
But, aside from that, some other issues I found were that sometimes hitboxes felt bigger than they should've been, like with the falling goo and the spikes, but maybe that's just me. I also think the goo sound effect is really grating after a while.
Now, I've gone and talked about all the negatives but I've failed to mention the good parts about the game! The art is the first thing that comes to mind, and it is really good! The second thing is the gameplay feels pretty good, jumping around is nice and the movement is good, I might complain that q and d are the movement controls but w/e. I also feel you did a decent job of designing the levels, but I think it might be wise to slow the difficulty curve, even if you ARE going for "NES - hard". I also think you should let the player learn how certain hazards work, like how the ceiling goop robot follows you.
All in all, I hope your not offended by the paragraph I've typed out, I know how it feels when someone criticizes your game and hasn't even beat it! But, I'm really trying to help more than hurt and I think you have a decent game here that just needs some polish!