We haven't had issues leaving the US from MIA. When we return via MIA there have been a few times we have had checked luggage and the waiting area was a madhouse and then you have to go through security again which can be a breeze or another madhouse. We have global entry but sometimes it is just the mass amount of people that stresses me out. As Dziadzi mentioned they had ~1hr 15min between flights, personally I wouldn't fly with less time than that. Prepare for the long lines and you'll be pleasantly surprised if you breeze through. Happy travels.
