The MXZ-SM48NAMHZ from Mitsubishi goes that low, without a failover. But even cheaper models from Mitsubishi go down to -15°C / 5F with ease, and you can easily supplement those with electrical heating in areas where temps below 5F are rare.
The only problem is that the outdoor unit needs to have a defrost occassionally once it gets too cold to remove ice buildup. This lowers the efficiency somewhat but they remain far more efficient than just burning gas until they stop working altogether. But the amount of gas saved over every single other day of the year would mean there is more gas available at the power plants to offset it with proper space heaters.
Modern heat pumps have a heater in the outside unit which keeps it just above freezing when in use.
Yes, it's possible for heat pumps to operate in sub-freezing temperatures. But can you find me the efficiency ratings for the Mitsubishis? The documentation on their site only lists it for down to 14F, while listing running for 5F, 0F, 05F, and -14F.
The point still stands once you get below freezing heat pumps become much less efficient and start up their own internal heaters to work. That's why I originally said the price of heating with them spikes.
Normally a heat pump can pull heat from the outside air, but once it hits freezing, the pump needs the internal heater to kick in for it to function. But there isn't any magic happening. The heat pump is running an electric heater just like a small space heater you could have. But those small space heaters use 1,500W, now scale that up to the heating needs of a house.
In cold climates, you are still looking at needing gas so you can run a hybrid system which would have major cost savings versus electric alone. I shudder thinking about the power needs for the utilities for non-hybrid systems along with charging electric cars. With cooling in the summer there's usually a pullback of demand during the night, in the winter it would get colder and needs more electricity to heat while needing to also charge cars during "off hours".
In the dark parts of the year, these colder climates have the majority of their days as completely overcast with maybe up to eight hours of daylight. So solar panels to help cut down on costs are out, and even if it was a cloudless day, those are normally much colder days and the solar panels will be covered in snow and ice.
In dry warm areas like a desert or California (but I repeat myself), solar panels and heat pumps for cooling make a lot of sense. Even at night when it gets down to the 40s they do an amazing job heating and are very efficient. But they aren't an answer to get away from gas in every scenario. Much like our autonomous cars that are just around the corner, throw them up against a harsh cold climate and they start not working as they should.