ABC is very well optimized for single voice traditional instrumental music. In that domain (and with a bit of experience at it), it's very easy to write and read ABC:
The header L: field defines the default length of a note. In the body of the tune, uppercase letters A-G are notes in the octave that starts with middle C, lowercase letters a-g are notes in the octave above that. A number following a note name is a multiplier for the note length. Bar lines are pipes.
The drawback to ABC is as you wander away from that original use case, it gets harder and harder to use. Here's snippet of an arrangement of Sleigh Ride I was working on a few years ago:
z +mf+ (+accent+e .^d/) z/ z|(+accent+e .d/) z/ z +p+ .^F/.=G/||[K:G bass octave=-1] +crescendo(+ .A .A .A .A|.A .A .A .A|+f+ +crescendo)+ +accent+ d zz +sfz+ G|
Studying it again, I can mostly figure out what it is trying to do -- z is a rest, things surrounded in +s are extra commands, so +p+ and +f+ are dynamics, +accent+ puts an accent over the next note, etc. It's still pretty easy to understand, but I'd have a hell of a time playing the music from that notation. And I coded up that ABC, if someone else had done it there's a decent chance I'd have to stop and look up some of their notation in the ABC standard.
The main drawback of ABC notation is that it doesn't capture polyphonic music well but by the looks of it, Alda's notation doesn't either. You can write polyphonic music in ABC but it is clumsy. Another drawback is that the ABC language has warts and has accumulated some cruft. Which is to be expected since the notation was invented in the 1980's and have been incrementally improved since. The main advantage of ABC is that tens of thousands of songs have been notated in the format and that it is well-supported by lots of programs.
Due to the nature of the data, it is very hard to create good textual representations. The most practical I have seen are piano rolls and MIDI event notation.
I've played with ABC / ABC.js to render sheet music in the browser before and I like it for transporting notation; but I don't think it fulfills the same use-case that Alda is attempting to hit.
The drawback to ABC is as you wander away from that original use case, it gets harder and harder to use. Here's snippet of an arrangement of Sleigh Ride I was working on a few years ago:
Studying it again, I can mostly figure out what it is trying to do -- z is a rest, things surrounded in +s are extra commands, so +p+ and +f+ are dynamics, +accent+ puts an accent over the next note, etc. It's still pretty easy to understand, but I'd have a hell of a time playing the music from that notation. And I coded up that ABC, if someone else had done it there's a decent chance I'd have to stop and look up some of their notation in the ABC standard.