I have a problem with midi on Mac OS 10.12.6 with a UR816-c.
the timing is all wrong… midi “stutters” and won’t play properly.
Updated Firmware and tried official 10.12.6 USB driver. Audio is rock solid, but midi timing is all over the place…
I play a chord, and the note ons do not all arrive at the same time.
Ironically its worse on older (Yamaha) keyboards.