You get lower efficiency for lower temps, so for a typical Swedish house you just need to consider how many days in the year you expect sub -20-25 temps. For e.g Stockholm this is only very few days per year so a good idea is to have a fireplace (which is nice anyway) as a backup in the very rare cases it’s -25 or colder.

In northern Sweden you can’t be surprised to see a month of -20 some years, so you probably want either a bigger heat pump, expect to use more direct electric in the pump (the heat pumps are air/water so the houses are heated with circulating water, not hot air) or something else entirely like geothermal.