YUL is my home airport. Construction at the US terminal is done. I guess it would be a good walk from the domestic terminal to US one but at least you can walk and don’t need to take a shuttle like when I had a transfer at Pearson a few years ago from domestic to US terminal (made it with a few minutes late but my luggages didn’t). Compared to the other airports I’ve been (Miami, Orlando, Vancouver and LAX), YUL is on the smaller size imo.
For the getting out outside, that can happen anywhere. It depends on how big the plane is. If it’s too small to fit the gates, you disembark on the ground. Happened to me once in Vancouver (Vancouver-Victoria flight).
If you have a Nexus card it is accepted at YUL and you go thru customs here which I personally prefer.