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.
Mitsubishi now has a HyperHeat+ unit that goes down to even lower temperatures than before, and that's before you add the heat strips for auxiliary resistant electric heating. Watch Matt Risinger's "Build Show" channel on YouTube for recent announcements from the International Builders Show 2023 on this.
Are you Canadian? I had previously looked into heat pumps for cold climates and thought that it was economically not a great investment and required a gas back-up.
No, Massachusetts, so probably warmer than you, depending on where you are.
Even in Canada, though, I'd look into whether heat pumps with resistive auxillary heat are economical. If it only ever goes below -20⁰F for, say, a week a year, you'd probably still come out on top. And you'd definitely come out on top environmentally if your electricity is mostly green.
And with heat strips, a heat pump unit can continue to provide heating even below -20. Heat strips do make the heat pump more expensive, but I would wager they're less expensive than having a separate furnace.
Have a look what's being used in the Nordics in Europe. Sweden has 90% share of heat pumps for residential heating systems for example. I've got a Panasonic that heats to -35°C as advertised by the manufacturer and-40°C as tested in the lab.
To be fair, though, space heaters are dangerous in older homes with unknown electrical system quality. You're running a circuit at just shy of it's max load for potentially long stretches of time. Any poor connections (cough, backstabbed outlets) along the circuit or damaged wiring could result in a fire.
Aux heat for heat pumps (not mini splits) usually starts at 5KW (4 space heaters) and is a dedicated circuit direct to the panel and sized correctly. It's also contained in a steel enclosure with strict rules about distances to flammable items.
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.