This is not a problem, more a question of style, but your bosses' attacks seem to rely more on speed and less on intricate bullet patterns than the attacks you can find in most Touhou games and danmaku fangames.
Possible issues I've found:
-You might want to add/improve deathbombing
-I think the invincibility after death should be extended. It's easy to die twice because the period of invincibility is short.
-Bosses appear very suddenly. It feels like I'm just playing the stage, then the boss suddenly appears on the top of the screen without warning, then it's battle time. At least have the boss enter the screen like Touhou bosses do.
-The stage 1 fairies shoot fast bullets if you fail to kill them in time. The bullets are fast, so it's hard to react to them if you don't know it in advance. Getting killed by a fast bullet in stage 1 may not be the most motivating experience to a new player.
-Cirno's last noncard is annoying with the icicles changing direction without warning
-Bomb damage carries over between attacks (if you bomb near the end of an attack, the bomb can damage the next card as well)
-For some reason, I find Mugetsu's noncards very hard to read, even on Easy and Normal.
-Some of the bosses have annoying movement patterns (too fast and too unpredictable. I know some Touhou bosses, like Futo in one of her Spell Cards, move around a lot, but at least Futo's movement is predictable)
- If you plan to make your bosses drop items after the end of a Spell Card attack when it's not the last card of the battle, you should make small pauses between attacks.
Anyway, I hope some of that helps and that I wasn't too harsh. It's pretty interesting to see someone code a danmaku game from scratch (no offence, Danmakufu users). I really like the effects. The game looks great
