Here's a list of bosses that don't work correctly with this right now:
-Gurdy (face isn't changed)
-Gurdy Jr. (face isn't changed)
-Duke of Flies (missing the yellow parts)
-The Husk (missing the yellow parts)
-Peep (works until eye pops out, then both the eyes and Peep itself revert to their regular sprites.)
-Pestilence (floating pixels on the sprite)
-Mask of Infamy (Heart of Infamy isn't changed)
-War (second form doesn't change)
-Death (second form doesn't change. Horse does, though.)
-Conquest (while it does it's charge attack, the clones don't change quickly enough)
-Brownie (Dangle form doesn't change)
-The Stain (it's side tentacles aren't changed)
-The Matriarch (face of it's Chub form doesn't change)