to be honest, the concern with delta/american etc from YYZ should be more about them changing the flight schedules at the last minute after you have booked, notifying you that you will now be missing your connecting flight and have to take the flight the next day, therefore getting stuck in your layover spot and having to either cancel your whole trip, take a refund and rebook on another airline (likely for 40%+ more due to the proximity of your flight date), sleeping overnight in the airport or paying for hotel/cabs etc to do a 13-ish hour turnaround in miami/atlanta etc....