[bombcar]

At a certain temperature differential you need to source "heat" from elsewhere, which is why up north you either need a geothermal heat pump (underground piping) or you supplement with gas or electric heat.

Some very efficient pumps can work down to 0 degrees, but most start losing efficiency at 25 to 40 (Fahrenheit).

For some parts of the world, a heat pump is perfect - pump in heat in the winter and heat out in the summer, but when the outside temperature can hit -40º Fahrenheit (which is -40º Celsius), you're gonna need something else.

[outworlder]

For the overwhelming majority of the world they are perfect. Even in high latitudes, they might only reach those extremes for a few days in a year, that's when you would require a backup.

Losing efficiency is fine since they are so ridiculously efficient to begin with.