Having dabbled in this technology over the years, both ground and air, ground is more efficient, but you cant extract heat 365 days a year as you get frost heave in the surrounding ground. Some big NHBC claims in from ground source systems being run all year round - they aren't as good as they say. The borehole ground source systems do counteract this but mega cost.So you have to move to a tanked solution, this is where heat pumps start to compete as they use a tanked solution as well. In fairness they can heat up a tank pretty rapidly, 30 mins or faster. Ground source much better than air source.
They just dont work in older houses that are not airtight and have modern levels of insulation as they cant keep up.
I agree we need to do things differently, and insulation is certainly one of them, but the sub 5 air tight box's you see on grand designs, all have to run MVHR systems to counteract the condensation be breathe out, cooking produces, showers etc. This is basically extracting the air inside your airtight box, passing it through a heat exchanger and warming up the incoming air being pumped in. All this uses electricity! Gone are the days of trickle vents, you need an electric motor running 24/7 now. We seem to solve one problem and create another.
The reality is, houses in the future wont be heated past 18 degrees. The sustainable systems cant get what most of us would call a warm house (i.e. 20c plus) Invest in a decent jumper people, good job global warming is here to help.