I'm not generally a fan of 'flappy' style games, so the fact that this held my interest was saying something. My remarks are purely of constructive criticism, to hopefully see a good game improve into an awesome new game.
Things I loved:
1) Dear god, THANK you for starting the game with an option to not have sound/music on. Every game designer seems to have a different idea of what 'loud vs normal' is and more often than not I end up wanting to mute games anyways. Automatic + for that.
2) Loved the visual aesthetic. It had a primitive 'old school' feel to it while nicely updating it with some gorgeous parallaxing. There were times I found myself admiring the background and getting distracted from the gameplay. The interface was also nice and easy to read. Lots of games go for 'overly stylized' and it can get difficult to read things.
3) Actually telling what the perks were. This is a big one. I hate the trend of "figure out this secret thing to get this award/perk/achievement." Just freaking tell us. Players will often gladly do the most difficult of challenges (one bullet, anyone?) if they just know what to work towards. This game actually tells you the necessary achievements so I can happily sink some time into trying to reach those goals.
4) Perk/upgrade system. Just...everything about that. I love games with upgrades and perks, and I like getting more powerful as a game goes along. Great job.
Things I didn't like:
1) This is a minor quibble, but only brought up since the rest of the art style is so great: The player character didn't seem like they had the same 'detail' as the enemies and background. They felt very flat and unshaded, which would normally be fine except that all the spikes, enemies, backgrounds etc had touches of shading. I'd like to see the player character get the same.
2) A few of the weapons. Some of the weapons didn't feel as useful or good as they should have. What would normally be joyous schmup-ish gameplay felt a little more 'luck' based sometimes. Like you had to make sure you fired your weapon at the exact moment of your arc in your jumps. I know that's some of the difficulty/drive of the game, but it's a bit of a deterrent for me. Again, I admit I'm not big on flappy-movement. I'd have enjoyed this more as a free-roamer but I totally get what the designer was going for.
3) This is the BIG one, the one I'd say they most have to say I'd like the dev to take to heart: Please don't go "the controls are in the tutorial." Just put them in the authors comments or better yet: In the pause screen. Sometimes you're in quick need of a double check on something...but going back to the tutorial level is annoying. It takes you 2 seconds to paste it into the screen under the game so we can glance at it.
Overall: Fun! I look forward to seeing if this game series evolves a bit more!