I have an Electric On demand that I heat my shop with (radiant floor heat) and have had no issues with it in 10 years. From my investigations,
- On Demand water heaters should be sized by demand - making them very efficient (not a one size fits all situation) You can install one large whole house demand but it reduces the benefits and increases the total overall cost.
- Often multiple smaller electric units are installed at the point of use (each bath, kitchen) this lessens the lead time for hot water minimizing water waste and allowing for smaller elec circuits per unit .
- depending on the mineral make up of your well water it may be an issue with deposits building on the element(s)/exchanger.
- My heater has an output water temperature sensor, up to 135degF.
My opinion is they are superior to tank type heaters but may require different maint. My system is closed loop and filled with deionized water so build up form impurities is not an issue.
I would not try to use one on my well water based on it mineral content (Hardness Is greater than 10gpg, high in calcium content).