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Sort of... It's not so much down to it just not working as good at transferring heat, because the rules of thermodynamics applies... moving heat is moving heat.

But the devil's in the details. If it's below freezing the radiator will frost up and won't work very good. But that problem is solved by temporarily reversing it to heat of the radiator to melt the frost off of it. These systems do this automatically. Freezing temperature is 273 Kelvin, so there is heat outside even when it's below freezing so there's always heat that can be pumped, but there are limits to it.

You don't want to be dependent on a heat pump as the only source of heat for your house. But they build electric heaters into many models to handle those conditions. But obviously on really cold days that it needs to supplement the heating with the electric heater it's not going to be all that efficient, because you're running an electric heater on those days.

But most days it's not going to need to turn on the electric heater, and on your cool spring and fall days it won't even need to defrost. So when you consider it over the course of a year, the heating cost is way lower.