Every time I see someone wanting absolutes about chickens (or for that matter anything), I find that there are always exceptions to the rule. There are ways to deal with just about any environmental extreme, heat included.
I've had more trouble with silkies than any other breed having trouble with the heat. I think their feathers just make it harder to regulate body temperature. As it is I'm down to one silkie, and she's the only reason I find myself putting frozen bottles out on hot days. Everyone else does just dandy with a bit of shade and plenty of drinking water.
Edited because I felt the need to clarify that I'm down to one silkie by choice . . . I didn't mean to imply they've been dying off in the heat.
I do agree that there are ways to deal with any environmental extreme -- people raise chickens, a bird whose natural environment is tropical, in sub-Arctic Alaska. But the only breed that I've had that had difficulty with the heat was my Cuckoo Marans. I lost three hens to heat, two one year and one the next year. They had shade and water, and none of my other birds seemed to have any problems. Maybe it was just that strain of Marans, I don't know. But I'd certainly take special precautions for them if I had them again, while I don't need to do anything special for my other birds.
I am new to poultry. Last summer I received a trio of 1 year old Cuckoo Marans hens and a roo. The heat was in the 100s for over a week, impacted by the stress of the move, I lost one hen one day 2 or 3. Did not have a problem with the others. Their coop was located in the shade, as well. I dunno .... just my experience.
Cornish rockX does not tolerate heat well. I move my meaties in the shop when it gets really hot if they are getting close to full size, as they are prone to problems in heat.