I see most of the people in the comments here have older model heat pumps.
Newer model heat pumps are quite a bit better than even ones made 10 years ago.
My dad had one put in two years ago for his garage kitchen, and it's worked quite well, even in fairly cold temperatures. Most importantly, that heat pump doesn't have an auxiliary mode, and yet it still works in sub freezing temperatures.
It would work even better if it were ground source, but those are both expensive and require digging up the yard.
This has been common in Sweden for decades, I had it in my previous house almost 20 years ago. If I remember correctly it was a 100m borehole. The only limitation is that certain soil/rock environments work better than others, so it's not suitable for all locations.
Heated our house in northen Sweden just fine in -20/30C winters.
Yeah, if I recall what I had read about it, the part that was new was that oil well drilling technology was being applied that could really expand the types of ground it could be used in. I think the equipment has also gotten more compact, so it's easier to drill in smaller spaces to be able to install it in more existing houses. I don't think it's really been any technology breakthroughs, but more like improvements in the engineering of it