Bay Area California. December has been highs in the 40s-50s, lows 30s-40s (Fahrenheit). I definitely benefit from living in a place that's not extremely cold or hot, which reduces our energy needs.
Most sources I've looked at say heat pumps lose efficiency below 40F outdoor temperature and only work down to around -5F. If I were to convert to only using heat pumps, I'd have extremely inefficient or straight up intermittent heat and hot water for about a third of the year.
Falling back on resistive electric heating drives your heat bill through the roof.
I'd definitely love to get a heat pump for the other 8 months of the year, but the cost is hard to justify when I still need to maintain some other form of heat anyway.