Awesome game! The core loop is really fun / satisfying, and the different enemy types keep things somewhat unpredictable. I found it a bit difficult to keep the momentum going after a bounce, even when I saw the enemy beforehand and expected the ball to bounce. I also experienced the overwhelming runaway effect when you let a few too many enemies hang around. My initial thought was to just have the game decrease the spawn frequency if there are a large number of enemies in the arena, but perhaps a more interesting approach would be to drop power-ups that could, for instance, increase ball size, or add a burst of momentum, or make it spin by itself for a short time, etc.
Great work! :D