I really enjoyed the art and sound design, but the thing that kinda ruined this for me was the seemingly random one-hit deaths I would constantly experience.
Like nothing would spawn for a while, then suddenly somebody would kamikaze me from off screen and it felt a little cheap. If the player HP was increased a bit, it wouldn't feel so harsh and would give a much better gameplay experience.
I also found that all the upgraded weapons were weaker than the primary weapon? Maybe that was just me though.
My main takeaway is that this game just needed more balancing, other than that, its actually pretty good. It runs smoothly and sounds great (even if the default volume is a bit too high).