While a lot of the problems of it comes from its limitations at the origin (think medieval musicians writing music) it is a very flexible and interesting format
What are the problems I see with it:
- Apart from the center notes, it's hard to know which note is which. I know that the second line from the bottom is a G, beats me what's that thing 3 lines above the regular lines
- It's hard to capture what's happening. Chords on top help
- Having to identify notes in G clef, F clef (and C clef sometimes)
- It's an absolute mess when you have lots of simultaneous notes
I know there are a lot of historical, instrumental or music-theoretical reasons things are like that, but there's room for improvement
I just wouldn't think of twinkle twinkle little star when designing it, I would think of something more complex (and Let It Be, props to him, is not very complex but also not very simple)
(And that's not touching the issues with the piano, that's another load of items)