1. Hard water (i.e. high calc content). Perhaps one of those nanotech self-cleaning surfaces could help there. This would otherwise interfere with heat sensors and reduce the effectiveness of heat radiation into the water, not to mention cause the stone to overheat if not cleaned properly.
2. Battery life. It would be a pain if the stone turned off during your bath.
3. Comfort. I'm not sure how comfortable it would be to sit on the stone by accident.
Reshared post from +PJ Rosenberg
Digital Pebbles – Keep your Tub Warm
Sometimes it feels good to take a long soak in your bathtub, but you probably find the the cycle of water turning from comfortably warm to tepid to cold a bit frustrating to say the least. For sure you can drain out some cold water and refill some water, but it is not only a unpleasant experience but also kinds of water waste.
Korean designer Sunmi Hwang came up with a brilliant and elegant idea – Warm On pebbles. Conceptual “pebbles” Warm On are sensor-fitted heat-radiating artificial stones. Dipping them in hot water, they are able to detect and “remember” its temperature. When the water begins to cool, the sensors are activated and turn the heating system. The water in the bathroom or any other vessel passes through an electronic “pebbles” and thus heated to a reference temperature, which is “memorized” transmitters. So from start to finish you can have the same water temperature without any guilt. I Love this idea!