I agree. I don't know if I would do that (place a heater directly on the substrate, e.g., sand.)
Sand absorbs heat faster than water, so placing the heater on the sand may cause the heater to switch off before the water temperature is the correct target temperature.
I don't construct heaters, and I can't tell you where every thermostat is located, but it seems to me that you don't want the thermostat near materials that heat faster than water, such as sand, since when the sand gets hot (while the water is still cool), the thermostat might switch off the heating element.
Suspended even a couple inches above the sand, but touching only water seems much better. Driftwood on both ends might be a better option, although I suspect the original design employed some sort of rubber clamps.
Also, if you have large fish, it might not stay put without a clamp.
