I tried an experiment this week during our cold snap. We've kept the door closed to the first floor ell (bedroom and bath) and let it run cold, because the Fujitsu wasn't sized to heat that space too. I opened the door early in the cold snap, and let the heat pump go, leaving it set on 70F. What I found was that overnight the main space went to 66F, and the upstairs and back bedroom were 3-4F lower.
My calculated heat loss in these conditions is about 24,000 BTU/hour, and the heat pump is rated at about 17,000 BTU/hour at about 10F. You'd think it would not be able to keep up. My heat loss number may well be too high, and the rated output of the unit may be quite conservative.
One other thing I wonder about is that even though the room was not at the setpoint it seemed that the unit didn't run on full output much. My system has the temperature sensing built into the wall cassette, so it may be sensing a higher temperature than out in the room. It may make sense in severe weather to set the thermostat up to 72F instead of our normal 70F. Unlike a boiler, these variable speed units taper off the output as the space approaches the setpoint instead of always running at full bore, so that may be a disadvantage of a smart unit - it's trying to stay at a more efficient operating point instead of making me as comfortable as possible.
Anyway, comfort trumped further experimentation and we closed the door again to the ell.