We have flown out of Toronto a few times because it was considerably cheaper ($200-$400/person). But we already had passports and were going to Cancun. I wouldn’t hesitate to do it again, it wasn’t complicated and the waits were minimal.
I live in Montreal and we got our Nexus card at the Champlain, NY border crossing. It was fast and it's so convenient! Nexus costs 50$USD/adult (free for kids) and are good for 5 years. With a Nexus cards, you wouldn't need a passport to fly out of YUL. You also go through US Customs at YUL so the flight is then considered domestic. Nexus also gives you access to a quicker line at security at YUL and TSA pre-check at many US airports.