I think that's less and less of a problem. We just installed a Mitsubishi Hyper Heat, that's supposed to work down to -20⁰F without any auxillary heating.
Many other units have auxillary resistive heating when the temperature are that low, which is obviously less efficient than the heat pump, but is still better than needing to maintain a fossil-fuel furnace.
Many other units have auxillary resistive heating when the temperature are that low, which is obviously less efficient than the heat pump, but is still better than needing to maintain a fossil-fuel furnace.