Here are two scenarios when a heat pump is harder to deploy: 1) Heat pumps need access to the outside cold reservoir, for example they don't work entirely inside a closed room, where an space electric heater would be fine. 2) If the temperature gradient is too large, heat pumps are less efficient than a regular heater. So many heat pumps fall back to a regular heater when the outside gets too cold. If it is very cold all the time, a heat pump would not make any sense at all. This is why they are deployed mostly in places with mild winters.