i will give this my best shot, as i have had many fish tank setups and now am doing snakes.

The issue here that i can see with the rocks is that, eventually over time of the rocks being heated for prolonged hours will eventually end up not just a single hotspot but most of the tank becoming hotter than you would want it to. As rocks do get hot and stay hot they also transfer that heat as well, because the molecular structure is dispating the heat so as you can see eventually the area that you were trying to heat has now gone from the size of the uth to a lot bigger. cleaning it wouldn't be that bad just run water over it to get rid of the washable deposits and than visually befor washing taking the hard deposits out, than cook to clean let dry and re-use, i guess to put it easier think about walking on concrete bare foot on a hot day just because the part your standing on is in the shade doesn't mean it cool. humidity wise i think the substrate of rock would matter one way or another unless using something like texas holy rock stones dont really hold water at all it would come down to the tank and how you have it setup.

..... like i said my best description of what i know from experience, i think its more to have a drasctic background to allow the snakes colors to come out better.... but that is me