Heat rocks have all kinds of issues. First they don't warm evenly. One part may be cold, one part warm, and another burning. Snakes and other reptiles won't always get off a heat rock that is burning them fast enough or at all. I have heard of snakes and lizards nearly cooking themselves.
Also, they can cause fires, since they don't burn evenly. And lastly, because you have to trail the heat rock's wire into the cage, this can leave a gap at the top of the cage, giving you snake ample space to escape.
When I get my daughter's snake, I plan to use an under the tank heater.