combination of humidity and temperature that takes down their efficiency.

I had heat pump climate control for 20 years in China (Shanghai), it was next to worthless in the cold, humid months like January. Barely kept the house 'warm', let alone comfortable. We didn't have any back up.

But you can usually go down to 20F and 60% humidity and be warm and comfortable. I don't care if the house gets 'toasty', one of the reasons I hate pellet stoves and wood stoves. I set the thermostat at about 68F and that's where I want it, and a heat pump delivers that without much trouble.