While I have never flown through BOS, my observations about the US travel system is that the scrutiny between US-US and US-CAN is no different - the TSA still cuts locks open to look at bags, you go through and take your shoes off because the 'metal' will set the machine off, etc. That said, you will have one additional card to fill out for Customs, but that's on the plane.

Air Canada recommends 1h30 to YVR, or 1h in advance to other destinations, but with the staffing cuts at AC (doesn't UA handle them in BOS?) it might be prudent to say 1h30 for all flights. Hope it helps

Just my 2p.

O