game is awesome! i have 2 suggestions to make items easier to use:
1. ability to select items with mouse scroll wheel
2. show the item name when it is selected
this is very fun and very impressive, but there NEEDS to be sync correction during a stage. one lagspike in the middle of a stage and the timing is permanently off.
i've also noticed there are no "You" indicators, which would help refresh the player's memory of what they're doing.
(global audio offset calibration would also be huge, but an unfortunate logistical nightmare for a game with reactive audio like this)
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!