Rheostats work really well as long as your room temps are completely stable. One of the breeders near me uses only rheostats and almost never changes them but his rooms are at 80º and change only a degree here and there. If you have variable room temps rheostats need to be adjusted as soon as the room does. It generally creates a seesaw effect temps too high, adjust, too low, adjust,and back to too high. Just be aware of that problem especially using tubs! They have no thermal mass to create a buffer between the heat and the snake, if the heat pad is at hits 99 the tub will be 99 a second later, unlike a tank that takes 2 or 3 min to alter. Sudden spikes can cause issues in tubs that are not noticed with other systems. Tubs are difficult to get a true thermal gradient in tanks are not so difficult due to the way glass heats and cools temperature gradients becomes much more difficult. I have not managed a gradient in my rack I only have a hot spot and a cool spot no real gradient.
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