the game really doesn't like two buttons bring held down, which makes 16th notes very frustrating to hit (see: the 1st line of the boss stage). parappa 1 and old versions of FNF had this issue as well.
as other people have said, the reaction time is way too fast at points. i think you should take a look at umjammer lammy's scrolling button display, as it's much easier to anticipate your next turn.
the art style, music, and concept are solid though, and i hope to see more in the future!