Not a big or critical one, but you cant currently see if a note was played again, when this note is in hold state. So, when C1 gets played, C1 become yellow, but when C1 gets played again without a note off before it stays at yellow and we got no visual feedback of the second note. Maybe a slighty animating (fading) could be added on every note hit, so the second note be more visible on the piano keys.
Same with playing phrases i would welcome if there would be a second color showing what is playing inside the phrase…