I like the sound effects and gunplay here. The menu design and cutscenes are also pretty cool.
The first thing I noticed was how empty the levels were. Sometimes I just wandered around, hoping to find an enemy to fight. I would recommend either making the levels smaller, or adding more things to do in them.
It also felt pretty easy. Most of the times that I took significant amounts of damage weren't even from enemies; it was from accidentally falling into pools or shooting explosive barrels (which seem to have GIGANTIC areas of effect).
It looks like you can cheese the rocket launcher boss guy just by shooting him quickly with the assault rifle. He becomes hitstun and doesn't move much.
I'd also recommend having a tutorial, or at least a single screen showing all the player's available actions. I didn't even know rolling was a thing until AFTER I finished the demo and read some of the other comments here.
Some of the enemies (especially the goo blob guy) wouldn't do that green blood splatter animation when they got shot, which made it seem like my bullets weren't effecting them at all. If they have some invincibility frames, maybe make them blink or something, so it's more obvious.
This was fun. Thank you for posting it.