I’ve heard that air source heat pumps can be very effective too, and far simpler to install. I wonder if there’s any discussion about converting ordinary AC units into reversible units, which supposedly is basically all an air source heat pump is.
Exterior heat pumps used in humid or below-freezing environments need some kind of defrost mechanism, just like a refrigerator. Otherwise your coils end up clogged with ice and the system stops working. The compression cycle is indeed symmetric, but the second order design constraints are quite different.
Modern setups are really good about managing frost buildup and can be more efficient than resistive heat down to -20C. Waste heat from the compressor is often used for the defrost.
