ABC is most efficient when writing out single-line unharmonised melodies, so it is ideal for folk tunes (which is what it was designed for). It can handle more complex music but rapidly becomes unwieldy, and there are better alternatives if that is the sort of music you want to write.

The problem with most such more expressive alternatives is that they aren't as easy to distribute widely as ABC. Some are binary, which creates its own problems, but even the structured ASCII ones have pitfalls. Of the ones I can think of right now:

Bagpipe Music Writer and PiobMhor: OS-dependent.

Guitar tab: never really recovered from OLGA getting shut down - the sharing community isn't what it was.

Diatonic accordion tab: seems to be the perserve of a few sites who keep the display code to themselves.

MuseScore: has a centrally managed code base, which is great for keeping online and standalone versions consistent, but what happens if the mothership fails? And the public archive of music is small and rather crap.