Well best I can tell when GBC uses the high color mode (static backgrounds with higher color output than regular screens) they are often glitched look like a bunch of bars, or 'smeared' or completely missing. This usually only effects title screens or cutscenes the games that have them. Certain games with video cutscenes might be missing as well.
Donkey Kong Country GBC title screen is glitched, I didn't play long to see if it effects anything else.
Crystalis has some a couple of glitched images in the intro/title screen. The game itself 'shimmers' as you move (everything on the screen looks like its shaking).
Alone in the Dark: The New Nightmare, a game that pushes the limit of the GBC's abilities and primarily uses high color mode for backgrounds has glitched images.
Tomb Raiders, which use primarily high color mode for its cutscenes is completely glitched.
The world map on Lufia GBC is glitched and missing most of the overworld graphics, only the water shows.
http://www.racketboy.com/retro/nintendo/gameboy/game-boy-games-that-pushed-the-limits-of-graphics-soundI couldn't get Konami Collection 4 to work (that's the one with the Castlevania II: Belmont's Revenge DX colorized version). It just hangs on loadup.
Without proper testing, I couldn't say which games are worse, than others, I'm sure some are still playable, but you'd have to get past the cutscenes into the main game.