Other posters are correct that water can transfer heat more efficiently than air, but they're missing or burying the lede: The temperature is very stable once

Other posters are correct that water can transfer heat more efficiently than air, but they're missing or burying the lede: The temperature is very stable once you dig down a couple of meters. Winter or summer, it's probably an even 10-20 C (depending on your specific location). The ground has tremendous thermal mass and can act as an enormous heat sink. Your little house-sized heat pump isn't going to vary the temperature very much. You can optimize your heat pump to the specific temp range and dump/draw as much heat as you like year round.