We have gone several times during the summer. This year we are booked July 18-30, smack in the middle of summer.
I would say that if you are into commando touring, the summer heat will make it tougher along with the summer crowds.
But if you are willing to relax, enjoy the sights and find shade when it presents itself, you'll have a great time.
I think different people have different tolerances to heat. We don't find Orlando to be unbearable at all during the summer. But we are used to hot and humid summers, so I'm sure that plays into it.