Is tethering/hotspot dangerous for your Android phone?
There isn't enough/real evidence suggesting that tethering is harmful to device hardware. AFAICT this is a feature-by-design that any android device (supporting tethering) should be able to handle without detrimental effects other hardware except perhaps that the battery may be degraded a little bit due to overheating.

(Just like using your PC to play high graphics games 24/7 with a gaming PC, even though the PC seems to be "overworking", you'd agree with me that's what its designed for. Isn't it?)