Having just done an LHR-ORD-ATL-MIA-LHR with inbound ATL leg cancelled in the air (returned; passengers dispersed over whatever available seats over the coming days) and outbound delayed by 2 hours 30 leading to a sprint from one side of Miami International to the other - don’t chance the summer weather.
Mornings should be thunderstorm clear, and the later arrival in the summer schedule make it difficult to find a matching connection (at least on AA).