My understanding is that for most residential heat pumps, the temperature needed to make the heat pump less efficient than resistive heating is so low that it enters a range that the pump doesn't even work anymore.

However, that's only a measure of efficiency. It could still be that the throughput isn't enough. A 30 kW resistive heater can ALWAYS output 30 kW of heat. But my 7 kW heat pump could produce anywhere from 14 to 30 kW depending on outside temperature.

Does that mean the heat pump gets less efficient as the outside warms? Because that would be fine. 7kW to make you home a constant temperature seems wonderful.