I say this as someone who recently helped undo the damage after an inept HVAC technician replaced someone’s thermostat (for $500, thanks California prices) and did it wrong. The quality of the instruction manuals is low, and the quality of online resources about how conventional HVAC controls work is lower.
As far as I can tell, the theory is that HVAC systems used to consist of a bunch of discrete components, strung together, with control power coming from one or two transformers or thermocouples [0], and essentially no logic in the equipment. A relatively modern heat pump may well rely on a mechanism in the thermostat to protect the compressor. The actual equipment contains nothing resembling a microcontroller.
[0] Yes, the actual power available from the equipment to the thermostat may be negligible, and old thermostats were mercury switches on bimetallic strips that were powered solely by temperature changes in the room.
Modern heat pump systems have an outdoor temp sensor and will avoid even trying the heat pump if it is too cold outside. Your installer should be able to adjust that setpoint. How low it can go depends on the refrigerant and rating... in the US higher SEER ratings often also means it can extract useful heat at lower outdoor temps.
tl;dr: The heat pump function is a trivial bit of material addition to a standard A/C and saves you gas/electricity even if it is only used for part of the cold season. There is no reason not to use one.