I wouldn't worry about briars being in the heat, but I have read stories about cracks in briar when temps drop below freezing because of the difference in temp between the inside and outside of the bowl. But there no question that cobs are probably the most indestructible of all the materials in either cold or heat. That's my experience, maybe other have different opinion.
Briars, cobs, Meers, clays, and exotic woods all work for me when the temps are temperate!! When I'm working in the ol' yard/garden, cobs and my plethora of "yard" pipes are my go-tos!! :cheers: FTRPLT