Heating pads for human use will have at least one safety device installed, if not two. As already mentioned, many of them will turn themselves off after a certain length of time, and in my experience, most home electrical heating devices also contain a thermal fuse that is designed to cut off the electricity if the internal workings get too hot. Unfortunately, these are "one shot" fuses, so if it goes, you either take a trip to the local electrical supply store and get a replacement, or you go buy a new electrical appliance.

UTH's will vary in how they're supposed to be used. One brand that I have requires open air beneath it so that it does not overheat and damage itself, while the other recommends insulating beneath it to direct the heat up into the enclosure. I guess it all depends on how they manufacture them. At any rate, mine have been running steady for weeks with no problems.