Hi! Thank you very much for your feedback! :)
There's actually no collision: only the timing relative to the music is checked, the visuals are only here to help. Usually playing on beat should work, but I admit that the window is pretty tight! I didn't want to validate input that felt too much "off", but maybe a larger margin would be helpful. Unless it is a FPS issue in sandbox mode as you say. That might be the case if the bars movements are not consistant or if the music loop is off-beat. Also I used a larger margin in the jam version, maybe I should go back to that value if it feels more "fair".
There is an end to the game indeed! The red level is almost the last one! I'm glad that you enjoyed the game despite its difficulty.