Comment by kevin_thibedeau

Combining characters are trivial to deal with. Some of the new emoji compounding uses things like the color squares to alter an emoji and you are forced to have a table of which emoji are dual function to know if they merge with their neighbors or stand by themselves.

Nothing else in Unicode acts like that. You can't properly parse complex emoji glyphs from a random starting point because you need previous context to know how to interpret following codepoints. With combining characters you just skip ahead to the next non-combiner.