they're also able to work completely without electricity and fuel transport (depending on your situation), which is increasingly becoming a concern in some parts of the world.

my optimal setup is an air-to-water heat pump connected in parallel with a wood furnace fitted with a flue gas afterburner, feeding a hot water tank. we already have a big thermal mass in the house so the heat pump would keep the temp stable 99% of the time, but sometimes it gets close to -40 and then it's good to have massive heating capacity.