Belly heat is only bad for hatchlings because of the size of tubs used.
It's more difficult to achieve the proper temperature gradient in the small 6 quart hatchling tub, so that is why back heat is preferred there. With belly heat, heating up a 6 qt tub can mean just a hot spot and no 'cool side'.
If you were to use a larger tub (i.e. 12-15qts), using belly heat is possible. But I believe the 6 qt tubs can be used if you don't put the tub over the flexwatt all the way. It really depends.
Personally, I use belly heat for my sub adult/adult snakes (28qt tubs). I really like back heat, but it didn't work out well in my house.
I have a room temp of 70-75 degrees. When using back heat, the flexwatt was 120 degrees to achieve a 90 degree hot spot in the tubs. That's because back heat heats up the air rather than creates a specific hot spot like belly heat.
Flexwatt isn't designed to run over 100 degrees so I was rather uncomfortable with it running 120. I switched over to belly heat and in the same room, the flexwatt only had to run about 98 to achieve a 90 degree hot spot.
It really depends on how warm your room temperature is. If your room is 80+, back heat works well. Anything lower, it doesn't work very efficiently or safely.
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