true some are on wood, fan is top side so I dont think it makes much of a difference vs the ones directly on metal rails, considering the high ambient temperature...in winter all will be fine. And they run a bit more than 80%...1000w evga's GQ, solid PSU, running anywhere between 750 and 930w depending if im dual mining or not. (at the wall, 120v, so on 240v its a little less due to better efficiency). Breakers all run at less than 80% all up to code, but the PSU's themselves dont really need to if they are quality (of course you are correct lifespan with high temp is shortened). Warranty is 7 years though...
generally they are heavy duty, but to be specific it takes only a single electronic component (like an IC or a capacitor to fuck up) to get in an uncomfortable position to accumulate heat and it can cause some weird behavior if it did not die a clean death.
i always prefer a clean death because it is very easy to identify the cause of the problem (stability issue), rather than finding the cause of the problem by the process of elimination..while weird problems takes time and effort to identify..like my 1000w seasonic PSU (I mentioned up thread) which causes restarts if it powers the system and gpus but as a secondary psu, powering only 2 gpus it was very stable.
there is a side of seasonic 1250xm that gets hot like when you touch it you would remove your hand immediately, that side I would not dare lay it flat on a surface...other PSU are of different make and build but still..you get the idea.
Depends on load. If he does 80 percent 24/7/365 it needs decent air circulation
it is still best to do a "good practice", as a crypto miner sometimes we switch algos often/seldom/always and consume higher power..by doing so it may not come to our minds that we should give more air circulation to the psu, and by doing a "good practice" we can prevent them from happening.