[BorgHunter]

Heat pumps are much more common in warm areas than cold ones, because the difference between an A/C and a heat pump is really just the ability to reverse the refrigerant flow, and they're very efficient at heating in mildly cold weather. I grew up in Florida, and pretty much every house there had a heat pump even thirty years ago, with electric resistive heating that kicks in when ambient temperatures drop below 40F or so. Where heat pumps don't work so well is when ambient temperatures are very cold, which is why adoption in northern states has been much slower.

EDIT: My grandparents' house had a thermostat that looked like this: https://i.ebayimg.com/images/g/uqsAAOSwTVlbyNN9/s-l1200.jpg They would call very cold (for Florida) weather "blue light weather", because the blue "aux heat" light would turn on on their thermostat, indicating that the system had switched from the heat pump to the resistive heat strips.

[Detrytus]

My sister just got a heat pump installed in her new house in Poland, where temperatures occasionally drop to 0 Fahrenheit. I wouldn't say they only work in "mildly cold weather" - as per new EU policy heat pumps will be one of the few legal heat sources, even in countries such as Sweden.

[BorgHunter]

Modern heat pumps can work in very cold weather, but they're much less efficient, which is reflected in their COP numbers. In my house in Chicago, we have a hybrid system--the heat pump works down to 20F or so, and we have a natural gas furnace for colder times. Natural gas is very cheap here, so this is the most cost-effective solution at the moment. I'm very eager to electrify and remove my dependence on natural gas, but I think it will be at least a few more years unless there's some breakthrough in cold-weather heat pump efficiency, or an enduring spike in natural gas prices--last time I did the math, the breakeven point for electrification here is around a COP of 4, which no heat pump can do at typical Chicago winter temperatures.

If I were building a brand new house, I probably would do it 100% electric. But most people here already have natural gas furnaces, and when they reach end-of-life they're usually replaced with another natural gas furnace. Hybrid systems like mine are catching on, but it will be a while before 100% electric is commonplace here.

[bluGill]

I consider a winter where the coldest it gets is 0F a mild winter. The important thing isn't average or normal it is the worst case. I've personally seen -25F here in the last 10 years - it was only one time and lasted about a week, but that means the HVAC system needs to work down to at least -25F just in case.

I don't know what the climate is like in Poland. Maybe 0F is as cold as you ever get and you are okay. Maybe your system will work to -20F even though you haven't tested it. But your might have a system like mine that while it can deliver heat at 0F, it is sized such that below 30F it can't deliver enough heat (I have the backup system for those colder days)