Air source heat pumps are great but do lose efficiency as the temp drops for heating mode. Most traditional heat pumps are either split or package systems. The heart of a heat pump is the evaporator and condenser coils, their main job is heat absorption & transfer whether in the cooling mode or heating.
An air source heat pump typically has supplemental electric heat strips that serve 2 purposes. 1 they provide additional heat when needed in really cold temps. 2 they provide a buffer when the condenser goes into defrost. Moisture in the air and cooler temps will cause a condenser(outdoor unit) to frost up, hence the need for defrost. Some condensers use a combination of ways to sense the frost buildup. Temperature and humidity sensors, some also detect the higher current draw from the reduced airflow on the outdoor fan motor. In cooling mode if an evaporator coil(indoor unit) freezes or frosts up it is typically either a dirty coil restricting the airflow or low Freon. On a heat pump low Freon will cool better than it will heat.
Electric rates compared to other energy sources especially relating to a heat pump are usually cheaper. It always a good idea to have a backup source when the temps hit sub zero. A lot of people want to switch their thermostats to electric when the temps drop. The only time that should be done is if the condenser quits working and then the electric heat strips are used as an emergency. It has to get very cold for a heat pump to quit producing heat. The temperature difference between the supply and return is typically 16-22 degrees, even if it's only 2 degrees because of extremely cold temps it's still heating which can seem deceiving. When it gets that cold the backup electric heat strips will come on to supplement as needed.
Dual fuel systems are nice but they're expensive also. By far the best heat pumps are water source heat pumps. Whether they use a closed loop ground system, a well, lake etc. They basically eliminate the condenser so to speak, eliminate the need for any electric backup and run nearly 100% efficiency regardless of the outdoor temps. However they are insanely expensive. Met a guy about 2 yrs ago who had 1, roughly a 6000 sq ft house on new construction, he claimed it cost him 80k.
About a month ago I had my heat pump changed out from a 90's model 10 seer American standard to a 16 seer Trane. The installers did ok but didn't have a clue on adjusting the variable speed indoor fan. I went through the manual and got it adjusted to my satisfaction which I'm more than happy with now. Have it setup so the indoor fan only blows as hard as it needs to, much better than my old system by far with good temperature drops between the supply and return. Also went with a slightly lower kw electric heat because of the higher efficiency unit. For a changeout it was more than I wanted to pay but ended up costing $7150.
