It is so bad that it motivated me to find this forum, make an account and post about how bad it is
It's bad enough that I have given up on trying to play music until it gets fixed

Going from 12 buttons on your hotbar (Plus up to 12 above it) to 8 buttons (And 5 flats and sharps) on this new UI means your range of available notes at any time is substantially reduced. From up to 24 notes down to 13. That is almost half your possible hotkeys to play notes with just gone
Being locked to octaves and having to fiddle with an extra key to go up or down an octave is a massive pain, breaks the flow of your music, and renders playing even the simplest tune a massive pain in the rear section
Being unable to bind individual notes to any key means we can't even work around this, as rebinding one note will also rebind that same note one octave higher or lower

The only good things are new instrument choices. But what's the point of new instruments if you can't reasonably play them?