Aren’t there other supplemental heat options, too? So if it’s “too cold” they can put a gas or electric warmer on the system to bring the temp up. With their 200%+ efficiency it would be like running a space heater.
Yes, I have an air source heat pump (basically a reversible air conditioner) with typical coils in an air handler with electric resistive heating coils as backup. Essentially, they can work with any backup system, whether it’s an electric or gas furnace, or other space heating units elsewhere.
Ya you could… you’d be better off though just getting a system that suits your location though, but the initial cost is usually the biggest issue. I live where it commonly gets in the -30 to -40c temps, and I know of 25,000 square foot shops with geothermal pumps that stay nice and warm all winter.