Air-source heat pump efficiency really takes a dive in freezing conditions when you need it most.
Ground-source heat pumps are difficult to get right, expensive to install%, and can be painful to get planning approval for^. Plus, if you have the land for a system, you often have the land for a small woodlot.
% You'll need more land if you have to trench instead of drilling a well.
^ Some areas don't allow direct expansion, so you have to do an inefficient heat-exchange. Running open-loop would be efficient and cheap, but districts can be very afraid of groundwater contamination.
Ground-source heat pumps are difficult to get right, expensive to install%, and can be painful to get planning approval for^. Plus, if you have the land for a system, you often have the land for a small woodlot.
% You'll need more land if you have to trench instead of drilling a well.
^ Some areas don't allow direct expansion, so you have to do an inefficient heat-exchange. Running open-loop would be efficient and cheap, but districts can be very afraid of groundwater contamination.