@Voltaire  
I guess it’s true in a way. The 5th bosses usually have some special ability or peculiar trademark spell cards, but the 4th ones set themselves apart in different ways. Like how Patchouli in EoSD has different lineup of spell cards depending on your player character, or the boss being different altogether in IN for the same reason.
 
Recently the 3rd bosses got weirder too. What’s with Unzan in UFO and almost unkillable Jiangshi girl in 10 Desires that likes to stall till her card’s Time Out.

@Gomenasai  
Dunno about that. I always thought the stage 5 bosses were the gimmicky ones. (Though they kinda toned it down for later games)
 
6th game: Sakuya’s time stop spell cards.  
7th game: Youmu’s pink screen bullet time.  
8th game: Reisen’s bullet displacement spell cards  
9th game: Komachi firing coins in a circle pattern.  
10th game: 2P Reimu and pentagrams  
11th game: Orin’s Kamikaze zombie fairies that respawn.  
12th game: Shou’s Frickin’ laser beams.  
13th game: Arrow shaped bullets.

Silly Prismriver ponies; you don’t need your hooves to play your instruments~

I know they’re Touhou characters, but for some reason, I can’t help but think “musicians of Azathoth.”

The 4th boss is always so gimmicky for some reason. Always, in any TH game.  
This one’s gimmick being 3 characters participating in one battle, that is.