If kept with a thermostat, the likely hood of the flexwatt or UTH failing is low (only if you keep the t-stat probe directly on the UTH!).
Even if it were to short out and cook your snake, if your checking on your snakes regularly hopefully you would be able to catch it or the snake would thermoregulate to the cool side of the enclosure.
Back-heat is another idea and is often used for baby racks.