I have heard of snakes being burnt by both (UTH and heat rocks). I've read not to use a heat rock at all. Also, more often than not, if the UTH is placed directly onto a glass/plastic surface, it needs some type of controlling device to regulate it (or have another form of attachment, such as to a tile that is not directly touching the glass). If not, the glass gets extremely hot to the touch, and just because there is substrate above it doesn't mean the snake is protected (since they burrow).
A UTH only needs to be warm enough to heat the top of the substate to an appropriate temp. For my snakes here in California, the heat pad is directly glued to the glass and feels like it is barely on at all.
This was a long-winded answer, but I wouldn't want to know at what point the snake is actually burning, much less risk it. Although, I am sure you were mostly asking from a curiosity standpoint, eh?