Yes, good points. And you're absolutely right, as the ambient temps drop so does the efficiency of heat pumps. Geothermal has the advantage of having very stable heat source temps. Below about 6' underground the temperature stays about 55*F year round in most places.
I'll convert KW to BTUH for electric (resistive) heating and show you how that compares to a Misubishi ductless system. Because your concern is heating I'll use one of their Hyper-Heat systems as an example. A typical heat pump starts losing capacity below about 45*F and produces little to no heat at 0*F.
Electric heat: KW=BTUH/3410. We'll use 38,000 BTUH to compare. KW=38,000/3410
=12.063KW
Mitsu PUZ-HA36NHA2 (3 ton Hyper-Heat unit):
At 47*F ambient 38,000BTUH=3.150KW (about 400% efficiency)
At 17*F ambient 38,000BTUH=5.400KW (about 225% efficiency)
At 5*F ambient 38,000BTUH=6.100KW (just under 200% efficiency and still 100% capacity)
Below 5*F capacity finally begins to taper off, but it is still 31,160 BTUH at -4F*. You have to go geothermal to get better low temp performance than that. And it will cost you many thousands of dollars more than the Mitsu system.
And, just an FYI, things are even brighter on the cooling side. Some of their systems go up to 26 SEER. And remember, that's with no duct losses. Couple that with the fact that you only heat and cool the rooms you occupy and each indoor unit can automatically vary it's capacity to match the requirements of the room, and control humidity. Show me a room by room zoned geothermal unit for less than $50k with virtually no maintenance requirements and then we can talk.
See why I say this is the future?