Today I successfully flew United Express from EWR to YYZ using only my NEXUS card.
United's checkin machines do not accept a NEXUS, I had to flag down an agent. She at first didn't think I could board without a passport, but when I mentioned the NEXUS is on United's list of approved IDs for international travel, she went behind the counter and double checked. She came back saying that yes, I am fine to fly with only a NEXUS on United. We went through the checkin process and she overrode the need for a photo of my passport, as the machine insists on, though we still did need to input the passport number and expiry date. She then entered my NEXUS pass id into her own computer to finalize my checkin.
I qualified for TSA Pre on my boarding pass as well. TSA accepted my NEXUS without a second glance. I was prepared to have to explain NEXUS to the gate agent when boarding, but she had also seen it and accepted it without issue.
So yes, on United and Air Canada, you can fly to/from YYZ and US airports with only your NEXUS.