Nothing too special, but I can see some charm in the chosen mechanics.
It's quite interesting to have such a rapidly regenerating health to recover from single minor blunders. Surprisingly enough, regenerating and limited ammo counter acts this by making it harder to bounce back up after using ammo.
The pick ups and scoring seem arbitrary. Your ammo reserves are your lifeline, it's not something you'd expect with random supply. Randomized chances to gain score also undermine the the value of score itself.
Bullet hell difficulty is a matter of opinion. In this case, I feel there might be room for unique solutions with the way ammo and health are currently handled.