Depends on the working fluid they use , generally it's R-134a and that boils at -26 C so the closer outside temperature gets to that the slower the fluid boils and the less heat you get from the pump.
Generally they set the pump to just switch over automatically to resistance heat at a predetermined temperature usually around freezing (32F or O deg C)
If they use CO2 as a refrigerant fluid it boils at -109 F so even if outside temperatures get down to 20 F, in comparison that 20F looks downright balmy to liquid CO2 at -109 F. So in short , this heat pump would still be more than 100% efficient at this temperature so it would not switch to resistance heat until say around -25F temperatures were reached. But since CO2 operates at very high pressures, everything has to be built for high pressures which adds weight and a lot of cost to the system, so these are generally for industrial use
So in short, the lower the temperature outside the more the efficiency of the heat pumps drop off. The reported 400% efficiency occurs at around 55F and drops off from there to 100 % when you close in that low temperature cut off I spoke of above.
At 100% your tied with resistance heating so you might as well use that and not wear out the mechanical pump