I liked how the bullet upgraded in ways that weren't just it getting more powerful. It kept the game feeling dynamic.
You spend a lot of time shooting things off-screen and getting shot by things off-screen. It would have been nice if the camera had a wider view of the game and enemies only shot you when they were on-screen.
The sword didn't seem very useful compared to the bullet. You're better off shooting the aliens before they get into sword-range.
Making it clearer what the HUD elements meant in-game would have been nice.