Thought I’d share my experience. This may be a little delayed but I used the Mobile Passport Control app at EWR Terminal B back in April. The officer was surprised to see me as a Canadian Passport holder that I was entitled to use MPC. He had issues with scanning my QR Code and proceeded to manually process me.

He had asked if I filled in a customs form (luckily I had as backup), which I answered yes and proceed to let me go through. I had to ask him to stamp my passport before going.