[Syonyk]

The 0C limit was true in the 80s, but isn't nearly as true now. My unit, which is far from a top of the line heat pump, holds a COP >1 down to about -15F, or -26C.

However, you're correct about coils icing up. That impacts some regions more than others. I'm in a fairly dry high desert climate, and our coils don't ice up unless we have fog (at which point, yes, they ice badly). There are defrost cycles that reverse the unit and melt the ice, but it is a problem in climates where it tends very humid in the winter.

However, there are a lot of places where it works fine, and one can always use a dual fuel setup, where a heat pump is used down to whatever temperature it starts having problems, then switch over to something else (gas fired furnaces are the usual backing option) in the extreme cold. You still get the energy savings of the heat pump while it works well, but can keep a house warm down to quite chilly temperatures.

Of course, if it gets really cold, a ground source unit becomes worth looking at (heat exchange with the ground, either via a deep well or a bunch of coils under the yard).