The refrigerant used plays a big part in how low a heat pump can still transfer heat. The new HC refrigerants are good down to -30°C .... at that point, the liquid remains a liquid when sprayed from a high pressure nozzle into a low pressure heat exchanger circuit.
The critical part is airflow across the heat exchanger, if snow is blocking the air flow, or it has had water sprayed over it and that is now frozen, there is no air circulation to extract the heat out from, the refrigerant remains a liquid and the system shuts down to protect the pump, they are designed to pressurise a gas, they can't pressurise a liquid.
T1 Terry