Unless you live somewhere that (air, e.g. in an EV) heat pumps can't function at high efficiency. Tonight and tomorrow night will be -20F/-28C. Always good to have a backup plan, no matter what your primary heat source is.
My Vaillant air to water heat pump is "effective" down to -28C, and has a resistive heater element as a backup in case the COP value flatlines (as in if COP is 1, it doesn't matter).
My cheap air to air heat pump in the summerhouse (Panasonic HZ25ZKE) is effective down to -25C and has a COP of 2.22 there. Even at -25C it still delivers twice as much heat energy as the electricity consumed.
While we very rarely have temperatures below -20C in Denmark, i have yet to experience a "drop" in performance from it. Granted, it becomes a lot noisier in very low temperatures, but it "does the job".
I'll add that this being an older house (1970s) we have "other issues" that causes heat loss, so we usually run the log burner for supplementary heat during those few days of -20C. The heat pump can keep the house warm, but you can feel the cold "pushing in" from walls and windows (dual pane).
Sadly the heat pump has also kinda voided all attempts to renovate for saving energy. Our yearly heat cost (heating and warm water) is around €750, and adding insulation would cost around €3500, for a potential saving of around 10-20%, so a total of 20-30 years to earn itself back again.
Almost no one lives in a location where heat pumps are never (or even usually) inappropriate. Yes, it might get to -20 F, but how often does that happen over a winter, never mind over a year?
My area doesn't get that cold, but the insulation is so good that last year we accidentally turned the heat off for a week without noticing despite it snowing outside; our "backup" was our own body heat plus the waste heat from our normal electricity consumption (which also isn't high).
I've seen a demo house in Canada that had a bucket standing in the middle of a room with -20 outside. The bucket had been there all winter and it never froze, a single, huge candle warmed the house. It was most impressive. I never did figure out how enough oxygen made it in to keep that candle burning!
But it really made me realize that even though I'm used to brick houses and stone everywhere that that is a terrible thing efficiency wise. A properly insulated wooden house can indeed be heated almost by body heat and waste heat alone. The big loss is windows so triple insulated and properly mounted windows are a must for such a setup.
Modern air-to-air heatpumps heat at over 100% efficiency even at those temperatures, they are very widely deplyoed in the Nordics for heating. And even where it is sometimes that cold, most of the year it is warmer than that. Still yes, you should have another source of heat just in case.
