1. Game is beatable with pistol only. No point to have any other weapon.
2. Too many bullets in pistol which allows to just spam them. Works excellent especially on last waves when enemies run as bunch of small sub-waves and you don't have to care about accuracy: any shot will hit something. Just burst them with pistol and you'll have plenty time to reload before next sub-wave. I can imagine what's gonna happen when shooting machinegun.
3. Piercing bonus is just a cheat. You shoot once and put down like 5-8 enemies.
4. Enemies trajectory is too simple and seems to be not random which allows to just catch packs on few "death points" and put them down with one pistol shot.
5. Soundtrack has no any interlude. Fade at the end would work better than just stop and start again.
2 for programming + 2 for gameplay.