I could not get MIDI over USB to work. Ableton showed incoming MIDI signals, but would not recognize any notes being sent.
Also, as per a couple of the above, at certain times playing within a patch (seems to be expression pedal based vol/wah etc) audio will completely drop out and the only way to get any sound back is to restart Helix. During sound failure Helix still looks and reacts/switches as if it were behaving normally.