Warm climates are good candidates for heat pumps.
Heat pumps have more efficiency in heating mode when the ambient temperature is warmer, less efficiency when the ambient temperature is colder.
If the outside temp falls too low, the heat pump stops providing useful heat. This is why heat pumps are often supplemented by "emergency" electric heaters that kick in when the outside temp falls too low.
Heat pumps can be more efficient than electric resistance heaters because they only need to move heat energy around, rather than create heat. When the outside temp falls too low, however, it takes more energy to move around the very small amount of available outside heat than it would to just create the heat with an electrical resistance heater.
According to wikipedia:
This typically occurs around −18 °C (0 °F) outdoor temperature for air source heat pumps. Also, as the heat pump takes heat out of the air, some moisture in the outdoor air may condense and possibly freeze on the outdoor heat exchanger. The system must periodically melt this ice. When it is extremely cold outside, it is simpler, and wears the machine less, to heat using an electric-resistance heater rather than to overload an air-source heat pump.