I think ground loop heat pumps are good choices because they reduce a lot of the problems you mention, but yeah, for heating the cabin of a battery electric vehicle, they are useful in such limited circumstances, that they aren't worth the complexity and cost.A few heat pump complications from someone that considers himself a heat pump advocate.
Capacity: the colder it gets the harder it is to keep up with the heat load as there is "less heat" outside to move inside and your heat loss is going up at the same time, Delta t, or the temperature difference between inside and outside. You basically need substantially more capacity for heating then you do for cooling the same space.
Defrost: you do need to reverse the cycle to defrost the heat exchanger that is outside. Heat pumps work great if your low temperature is 40° Fahrenheit, they struggle when it's just above freezing and raining, the coil ices up all the time and so you waste a lot of energy defrosting it. Meanwhile, you don't want the passengers to get cold so you have auxiliary heat which is basically electric resistive heat in an EV.
Defog: you use the air conditioner to dehumidify the air to defog the windows, at the same time you need heat so that again necessitates auxiliary electric resistive heat.
Cost: larger capacity system, larger components, more refrigerant, controllers and valves for defrost and reversing, it all adds up.
Weight: no big deal in a stationary application but in an EV every pound counts.
Living in the Northeast, I am not opposed to heat pumps in electric vehicles, it's just that it's not the slam dunk engineering trade that everybody seems to think it is.