The musical symbols that change up the instruments are nice, but there's a problem. Do they share the same loot pool as the notes blocks? I think they do, which means the musical symbols, a cosmetic item, are stealing from my beat blocks, a necessary upgrade needed for all but the best players to advance. This may be a non-issue after a certain point, but when a run fizzles because no note blocks dropped, it becomes a point of annoyance.
Which is extra fun when I end up with a whole bunch of notes, and no beat blocks which which to hear the instruments I have been granted by rng. But there is probably the big thing. RNG.
This game seems less willing (in its current state) to offer meaningful choices. Shops would likely be better if they always offered a note and a heart. Or at least they always offered those things if you were low.
The general darkness seems like it is there to help the player's projectiles stand out, but that really doesn't help when the point of those projectiles is to find the enemies who are always wreathed in darkness, especially the enemies that fade to shadow and will hover over the player before popping back into view. Having a level submerged in darkness like that is best as a modifier, not a regular state of the game.
Choosing rooms is also pointless without the actual choice rooms. If you aren't placing the room that labels what the choices are, then you might as well railroad players through a single exit. Having three unlabeled and indistinct doors only means the player can leave the room a split-second sooner.
That's a lot of negatives, but the game is overall quite fun and addictive. It is enjoyable to play around with the placement of the noteblocks and the types of sounds they produce. This is a clever mechanic that is worth expanding upon. Being able to change the sound of the 8th note special abilities would the appreciated as a first point of attention.