The risk, especially with heating pad type units and those horrid hot rocks is failure of the thermostat. Many have no thermostat and rely solely on a single resitior in parallel to the circuit for control. Believe me when I say, the risk of failure, and even fire is high. A good rule to follow is just like with aquarium heaters- use one size smaller than what the manufacturer suggests.
I feel the safest way to use these products is to try to avoid direct contact with the animal. In Dumper's case he has the glass as an insulator, so I would not worry too much. The good thing about amphibians, unlike reptiles, is that they are often smart enough to move when they get too warm.
I am very sorry to hear of the loss of your pet, Tyler. (Do not tell anyone, but I get pretty choked up, even by normal, natural losses.)