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Not less efficient than just using electricity. At -20C you get 1:1 electricity to heat.

Most heat pumps in the US have emergency heating that’s basically the same as electric. I believe even my gas furnace has an electric backup.

It should be the same in Europe and it will work in case the temp drops too low.

Heat pumps are always at least as efficient as resistive heating, but do have less efficacy when the ambient temperature goes below their design parameters.

Standard heat pumps these days work great even at 5C, at least any one that I have seen.

The H2i line from Mitsubishi maintains 76% of its COP down to -25C (-13F).

Just the heat pumps sold to warm countries, others do ok in -20C and even below (eg COP 1.5 at -25C).