The game is really fun, but there are a few things that make it a bit frustrating. One issue is that the power-ups move around the screen, so I sometimes miss my shot trying to collect them and end up wasting a bullet. Another issue is that the fast enemies are very fast. I think it would be more balanced if they paused briefly from time to time. Finally, the waves with five enemies feel a bit overwhelming—I usually end up taking unavoidable damage because it's very difficult to deal with all five at once.
While watching the Game Over screen, I noticed there are even more enemy types later on that seem to have some of these same balance issues, especially their speed. I think the game could use a bit more balancing overall. Other than that, it's a good game. Great job!