That's true, but I would argue the efficiency with those latest units make them viable for most households in Vermont as the main source of heat for the majority of the winter/fall/spring, as the amount of days where it is truly cold enough to make a modern heat pump struggle are so few, one could easily switch to a back-up source of heat for those, yet still gain the efficiency gains for the rest of the year.

Modern heat pumps from Mitsubishi have such excellent efficiency, they're able to produce heat down to -22f.

To learn more about heat pumps, how they work, and why that 200% efficiency claim is actually true, I recommend checking out Technology Connections video on the subject.