The problem with heat is not the watch itself, its the battery. All lithium batteries will be badly affected if you stay all day in heat (over 30 Celcius). Apple doesn't recommend using any Apple device at over 35 Celcius (were you can get quick irreversible damage), but longevity of battery is affected before that. That's will occur with all electronic devices using lithium batteries, not just Apple devices.
You were facetious; but in some jobs, you couldn't wear any smart watch or you'd likely mess it up quick. I worked in a factory in my late teens (made cans of all type, which uses sheet metal) as a summer job and temp there was close to 40 Celcius for 2 months in the summer.