There are many 'subtle nuances' to factor in, in order to REALLY realize the benefits.
If you live in a relatively high humidity area, for instance ........ you'd be wasting a pile of energy on "defrosting the icebox". A heat pump produces 'coolth' on the outside coils, since it extracts heat from outside and puts it inside. This makes humid air lose water (sweat) where it's cooled down ...... and that freezes into a massive ice-cake on the evaporation coils outside. So ....... every so many hours, the unit has to stop and automatically melt off the ice (just like an auto-defrost on your fridge). This gobbles up a pile of electricity, since it's producing heat instead of extracting it.
I would think that location is also important. You need to circulate new air all the time ...... and not recycle air you've already used (outside). If your heat pump sits in a sheltered alcove, you'd be getting less efficiency than if you had it sitting on a rooftop. The "nook" around the heat pump, close to the ground ....... would be noticeably colder than elsewhere in your yard, because the pump keeps extracting the heat from that pocket of air.