When going USA to International via Canada, there is no customs or immigration to clear since technically you are not intending to stay (or leave items) behind in the country. Hence, the checks are very minor with luggage behind forwarded to final destination. That and MLLs are one of the few redeeming qualities of doing a Canadian connection. An hour is pushing it albeit as others have noted if you mis-connect it's on the airline to figure it out!

Certainly wouldn't want to spend more than 45 minutes in YYZ without MLL access, in which case it become a field day!

Safe Travels,

James