[Brian_K_White]

But 5f is still high.

This is actually terrible and supports their point. Heat pumps are not sized as if they were running at a mere 100% or less, they are sized for the btus they normally produce, and aim to run close to 100% of the time, meaning they will be too small and not be able to do the job when they drop below 200-300% efficiency, unless they are grossly oversized. That leads to short cycles which makes condensation and moisture in the summer and wear and tear all year and less electrical efficiency because starting a pump and blower costs more than running it.

Current inverter types that can run slow do allow them to be oversized but it's still not great. Those are more expensive and have more failure points in electronics too.