My recent two experiences with UA @ YYZ:

July 2022: Original YYZ-ORD-SFO in evening. check-in with machine, want to do Global Entry interview on arrival, end up it need 3 hours wait so give up.
then, our aircraft was stuck in PHL for earlier flight before coming to YYZ and flight cancelled. Rebooked to next day 7am YYZ-SFO nonstop by UA agent. Cannot stay in pre-clearance area so UA agent send us back into Canada and retrieve checked luggage in 15 minute.
the second day arrive airport at 3AM . after check-in wait in line for customs to open at 4:30AM. the security is so slow that it did not clear security till 6am. Then, receive flight delay message till 9am. End up the YYZ-SFO flight did not board till 11:30am.

after boarding, pilot announced crew overtime and flight have to CANCEL. deplane and change to another evening flight by UA App. short UA customer service line (when compare with AC, more than 500 people in line, with only 2 agents, some say they were in line for 5 hours) gave me meal voucher. end up successfully fly out of YYZ 5pm and back to SFO midnight via ORD. Check luggage successfully re-routed with us.

December 2022 (before the winter storm): noon flight YYZ-ORD-SFO, check in with machine. This time I have Global Entry. need to show your Global Entry card to get into the Trusted Traveler lane. Security is a breeze, US customs is a breeze as we are the only one using Global Entry. Flight depart on-time. Check luggage go through to SFO without problem