They sound like good idea on the surface, however a heat pump to heat water requires a lot of surrounding ambient warm air in which to draw that warm air from.
Many Villages homes have the water heater in a small conditioned room along with the air handler for the regular heat pump.
Also I understand the the fan is somewhat noisy.
Don't know how one would be installed in the newer homes with the water heater outdoors on the side of the house. Besides, the newer ones operate on natural gas.
Everything You Need to Know about Heat Pump Water Heaters - This Old House