I have lived in Cleveland, OH, Boston, MA, Atlanta, GA and now on the Gulf Coast.
There are big differences in the South between places like ATL, Raleigh, Nashville and more coastal places like Miami, Houston, etc. Summers in the more northern/southern towns do get hot with a couple of weeks of 90+, but they do not last that long and you do get an occasional front in the summer to break things up. In the more coastal places, summers are a solid five months. May to October are 90+ most every day and late July through mid Sept can be brutal.
But you adjust. When I first got back down to the south, I could barely make it through a 6 mi run in the summer. A few years later, I was doing 20+ milers in the summer with no problem. It isn't fun, but it is doable.
Endless 90+ degree days seems horrible when you come from the north, but it is actually just fine. You just have to be in the water during the hottest part of the day. In the late afternoon, the humidity drops and it is actually very pleasant out in the evening (bug spray required).
I would take the heat over the long, cold and dark winters of Cleveland, OH. Summer in the north is just too short for me.