Whole-house heating with resistive electric heating is going to get expensive, quickly. If they're serious about installing these in cold weather climates, they should have more information about the cut over temperature and the efficiency of the heat pumps at those temperatures.

Just for context, my heat pumps cut over around 40-45 degrees to gas-based heat.

You have an older system. New systems can provide 100% rated capacity well below freezing. Mine is -5F and not specifically a "cold climate" unit. The unit in OP seems to offer 100% nameplate capacity at 5F.

These are brand new heat pumps. I don’t think it’s possible to have high efficiency at cold temps because of the need for a defrost cycle. I just check and mine are around 2 COP at 8F and 5 COP at 40F.

Yeah, I'd like to see those figures too, and I think they might surprise you (and me). I don't have figures or references to hand here, but as I understand it modern heat pumps should be good down at least a little way below freezing. They're widely used and popular in Norway, for example, and the Norwegians aren't exactly new to cold weather.

High efficiency heat pumps with built-in defrosters (or "heating strips") can often provide full heating capacity (ie: 48,000btu for a 4 ton unit) down to -12f, with reduced output at even lower temps. The COP efficiency drops quite a bit though and it will eventually get to a point where it is essentially as energy efficient as a resistive heater. Whether it's more efficient than using gas or not is a difficult calculation based on your gas price, heat pump COP temperature curve and electricity price. If you expect to see <-20f temperatures regularly, you'll want/need a backup heat source anyway.