can’t speak to the reliability of dedicated bike computers, but I’ll add a data point that the phone I use as a bike computer has never overheated. i’d imagine as long as you’re moving at a reasonable speed the airflow would keep it cool. I don’t ride in sweltering heat but 5+ hour rides in bright california sun, with long slow climbs, has never killed the battery or overheated the phone.

the only time a phone has ever overheated on me was when it was sitting on the sun on a 110 degree day in Vegas, poolside. no air flow and extremely hot out. I would certainly fail before the phone on a bike in those conditions 😂