Air-air heat pumps have gotten a lot better in the last 10-20 years and a modern one by a reputable brand will absolutely still work at Nordic winter temperatur

Air-air heat pumps have gotten a lot better in the last 10-20 years and a modern one by a reputable brand will absolutely still work at Nordic winter temperatures. People buy those too, but not generally to heat entire houses. Air-water is not popular here, I know they exist on the market and some people probably have them but I don't think I've ever seen one.