"1) Air source heat pumps have become incredibly efficient even in cold climates.
2) Ground source heat pumps are complex and very expensive and probably best suited for large installs like schools.
"
This is not really accurate on the complexity/etc side. Expense we'll get to later.
A. The GSHP is not complex at all, and often much easier to understand than the equivalent air source machinery.
I could repair my GSHP.
B. The efficiency of air source heat pumps vary a ton in the field. This is not conjecture. This is backed up by years of research. Of course, unlike ground source heat pump (which usually contain tons of sensors and can tell you exactly how well they are operating in a lot of ways), air source heat pumps rarely contain any of the performance measurement sensors necessary to validate their efficiency in practice for your install, and they are rarely accessible. So you are limited to a small number of studies that bother to test them in the field by adding that kind of metering. Oh and manufacturer "self-reporting" about how good they are.
EER or COP of air source heat pumps in-situ can be easily 50% or less of claimed efficiency depending on real climate and how well designed they were for the situation.
GSHP are 85-99% of designed performance over a much wider climate range (again backed up by research, and with much larger sample sizes, since most GSHP have the monitoring sensors built in).
That's on top of usually much higher efficiency of GSHP over ASHP.
C. GSHP not amenable to the random "don't want to have to care about real engineering" HVAC contractors that most people deal with.
IE not even willing to do basic manual j load calcs.
They are only complex in this sense - most air source heat pump folks don't really have to know what they are doing, and it will often work okayish in terms of ability to heat/cool when they size it wrong. It will just be very inefficient.
If your ground source heat pump folks don't actually bother to do the work right and enter the calcs into the loop design program, it can fail.
Just about 100% of failed GSHP installs i've seen were done by air source contractors badly trying to moonlight because they thought they margin would end up better (and who were bad at ASHP as well).
Any good loop design program will give you the EER/COP of the system in your case based on historical temperature, along with actual yearly cost/etc.
So you don't have to guess whether it will work out for you (and it will be very close to reality).
It's also obvious if your contractor is giving you bullshit if you have a loop design report - IE if the page with the assumptions/data show clear nonsense.
As performance standards increase (IE California), or any form of real in-situ efficiency testing of air source heat pumps was ever required, you would see this "really efficient" argument disappear really quick.
(Heating has definitely gotten a lot better in lower temps, not gonna argue with that).
D. It is true that GSHP is expensive to install, but that's mostly a factor of scale for drilling, and not size of install or unit cost. It's never going to be an effective way to deal with a single 2000sqft house. It will be a much more effective (and space efficient) at serving multi-unit complexes, regardless of individual house size.
For example, when they do GSHP for a new construction community (out here they are doing 200 new homes for examples), it's not a lot more expensive than installing air source pumps.
For single houses, again, it depends on your real in-practice performance of an ASHP, which is rarely ever measured.
For a house i owned in MD, and compared to two ASHP's that were properly sized (and properly sized/sealed duct work/house/etc with actual leakage/r-value/perf testing), but just a really bad in-practice fit for my climate (neighbors had same issues, etc), it only took 5 years to pay back the GSHP cost (after tax credits, etc).
This is uncommon case, obviously. But i'll also say being able to set the house at 72 and leaving it that way forever is worth something too.
Ductless i've got no qualms with :)
I think most folks with tight homes would be much better off in practice with lots of ductless wall/ceiling units than standard ASHP/GSHP.