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.

Reduces your energy needs and makes a heat pump a complete solution.

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.