To be honest, when I saw the simple graphics I thought it would be one of those made in a day, unpolished submissions.
Boy was I wrong.
This reminded me of those demo discs you would get in the mid to late 90s. There always would be gems like this hidden on them. Simple (but readable, thanks to that) graphics, simple controls, and a hell lot of fun.
I liked how every boss was distinct, not only in how they look, and act (most of decent submissions in this jam have that), but also in the way do kill them!
If I had to point out any flaws I'd go with the difficulty level. Maybe it's a bit to high. Like a bit less bullets on screen at any given time, or a health pack a bit more often?
Over all- one of the better ones here!