So I looked into them because of your comment, and they have a very big drawback of taking a very long time to heat the water. That wouldn't work for me, our water usage tends to be very "peaky" - my system can heat the water almost as fast as you can use it in a shower.
Maybe with a very complicated system with an extra large storage tank, and perhaps integrated with the home heat (so a single heat pump). I feel like the tech isn't quite ready yet, and I don't like being a pioneer.
It seems like a no brainier if you heat with electricity, but not quite ready yet for those who heat with natural gas. Right now it seems better suited for southern climates.
Personally I do not care about efficiency: I can't sell energy I produce so the point is just heating the water and the load profile of a classic resistance and the load needs of my houses match normally to heat without the heat-pump (witch have a far limited life respect of a classic resistence)...
I use the heat pump mode only when there is little solar for few days where there is not enough overall energy production, otherwise being able to produce 120-130% of the overall consumption (of course ONLY when the Sun shine, so NOT independent as some sell just playing with absolute numbers) why care about efficiency? I can't use the extra energy anyway...