The problem with heat pumps in cold temperatures is that the compressor (outside) unit has to go below the air temperature in order to absorb heat. I'm not sure

The problem with heat pumps in cold temperatures is that the compressor (outside) unit has to go below the air temperature in order to absorb heat. I'm not sure the difference, but I'd expect it to have to go to -30, -40, -50, at those temperatures its difficult to build mechanical devices that are going to work reliably for 10+ years.