I'll take a stab at it...
I believe the mechanism by which an RHP and a CHE transfer heat is similar (radiation), but an RHP is designed to be much more efficient. Therefore it uses less energy and its surface isn't as hot to the touch. The RHP is larger too so it can distribute the heat across a wider area. I don't have too much experience with CHEs, but the max surface temp of my RHPs is 160°F. Although prolonged exposure to that temp can certainly burn, since the RHP is mounted to the top of an enclosure, the snake will not remain in constant contact for very long if it does touch it, unlike with belly heat.
You ask whether an RHP is better than UTH for a T8. I think the answer depends on your ambient air temperature. My house is kept cold, so as you suggest, UTH does little to help that. For that reason, I went with RHP in my T8s. I use coco chip substrate and mist periodically to maintain humidity.
As far as how an RHP works - radiative heat transfer. The air itself is not heated, rather objects in the enclosure absorb the energy emitted by the RHP. However, as a result of the objects being heated, the air warms too.
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