I personally would not use a UGC, tape cassette case, or anything else other than a DS case to hold Gameboy, Gameboy Colour, and Gameboy Advanced games.
I like my shelf to look uniform (i.e. spines will match), so having Gameboy, and Gameboy Colour games in UGC while my Gameboy Advanced, DS, and 3DS are in different styles of cases I think would kill the uniform look of my handheld game shelf.
This is my personal opinion.