The camera knockback effect seemed great for feedback at first, and then it got really annoying as I started noticing it more and then my eyes got used to it and stop noticing it. I don't think I'd use that kind of effect myself, I think maybe having the gun itself shake would have been less-effective but more sensible choice.
The maps eventually got very hard to navigate and find enemies. Having arrows that point towards enemies that are off-screen and sit on the edge of the screen would be a great addition to counter that issue.
Other than those 2 minor issues, I really enjoyed the game.