Some parts of it are not super orthogonal. Intervals aren't visually consistent, for example (E -> F vs C -> C#).

Having to read notes that are up in the ledger lines outside the clef kind of sucks. You just have fewer visual landmarks to hang off of.

This is partly a consequence of the physical instruments we have and their natural ranges, but clefs as a general concept are annoying to deal with when you have to juggle them constantly.

IMO it doesn't do a good job of communicating dynamics and expression. The notation used is vague (accelerando... okay how much faster, over how many measures, etc).

Maybe I'm holding it wrong, but recently I transcribed some of my friend's songs and managing repeats that don't neatly fit into a rigid song structure is difficult without writing very "WET" music that takes up a lot of space.

In a more meta sense, most sheet music is garbage. There is no inherent pit of success when it comes to creating sheet music, and writing a very good, readable score is something only well-trained musicians and composers can do. Look at free sheet music or read notation written by amateurs, you'll see missing key signatures, misspelled chords, etc. Sheet music doesn't make any concessions for people who understand music as sound first, you have to adhere to tradition and theory to produce good sheet music. Lots of musicians with decent ears cannot do this.

IDK, this is me trying very hard to come up with reasons to hate on sheet music.

I am currently trying to learn to read sheet music, so not an expert, but at the same time all the pain points hurt more.

The fundamental problem, as I see it, is that music notation was designed for a 7 note system and modern music is a 12 note system. They managed to get all 12 notes in there, but it is more awkward than it should be, needing to notionally classify some notes(or bars, that's fun to wrap your head around) into a sort of second class status(flat or sharp).

Note for them like me learning this stuff. All 12 notes in common music are equivalently spaced and valid, The normal way to work with them is to pick a subset that sounds good together(a key) and mainly use those. then because the notation only naturally supports 7 notes you annotate the bars to shift their meaning around to match the key notes and for items that still don't fit annotate the displayed note to shift it.

But, this is the system we are stuck with, it works well enough and as I learn I fully expect to overcome then forget about the awkward parts.

Update: This just occurred to me as I was thinking on the subject. The only reason for a flat to exist(they are unneeded and redundant otherwise) is for when you shift a bar up, with a sharp, you need a way to shift the note back down, with a flat.