It not whether they can operate when it’s cold, but more whether they can operate EFFICIENTLY when could, plus, of course, the purchase price for these types of heat pumps. From what I can see, when very cold they’re better than resistance heating, but not by much, when temperatures are mild, they do much better. Even the government goal of 1.5 to 2.0 in COP means that the government knew there were limits, since ANY heat pump, even the cheapest, easily gets 3.0 or higher during their designed-for temperatures, roughly 45-50F.