First. Is it the temperature being too high where they may fatigue: I think the temps are manageable at that section of the exhaust system.
Too low that they would not work properly in scavenging the exhaust pressures etc. ie: pressure drop and exhaust speed slow down.

Corrosion gases or chemicals from exhaust fumes/gases that would make the muffs not last long at all.

I would like to make a set if you metal gurus think it can be done.