The heat pumps are very efficient. Usually a coefficient of production of around 3 to 4. It goes down a bit as the water temperature increases, but up to 60deg C it is up around 4.
The problem is in the initial expense. They are (or were, I haven't checked them recently) very expensive.
I see a few places up north are using them. I don't know why when it's so sunny up there you would think solar would be the obvious choice.
I was thinking maybe use solar/electric with some solar panels to help supply the electricity when needed.
Solar panels can still supply significant power even when it's a bit shady.
I like the idea of the heat pump from the point of view of the technical cleverness of them but I think they might lack a bit in the practical aspects.
As with all air conditioners (heat pumps) they have a limited life and are expensive to replace.
It means when either the water tank springs a leak or the heat pump wears out, you have to replace the whole lot.
I know they say you can just replace parts of it if it packs up, but the reality is, when it does pack up they will say it's cheaper in the long run to replace the whole lot. A bit like replacing car parts really. These days you mostly just junk the lot and buy another one. (Well some people do,.. but I don't.

. )