Probably a bit more nuanced than that.

A lot of the installers run small businesses. If one of these units goes wrong or if they do the install wrong because it's new to them, then that's lost time and lost revenue rectifying it.

This is it. They price installs at generally a single day for complete system swap for install time assuming ducts are reused and never want even spend the travel time coming back on another day in my area. If they do, it’s eroding the profit they expected on your job. They usually have special crews that only do installs and also generally like to keep to a small list of manufacturers so they can keep as quick and efficient as possible. New and unfamiliar tech throws a wrench in that.

I've installed two Nest thermostats and there's industry standardized color-coding and functionality and Nest conforms to that. (I don't doubt that you had that problem, but I think that speaks more to the incompetence of the original installer than to the complexity of installing a thermostat with a screen.)

I think it was more that the heat pump itself really wanted to be integrated with the Carrier Official Thermostat (which I think might have been an ecobee but can't remember), and the documentation on how to run it in legacy/standard/on-off mode was missing or confusing.

Inverter heat pumps shouldn't be controlled by dumb "smart" thermostats like Nest. They send only on-off signals, while a compatible communicating thermostat sends a numerical setpoint, allowing the unit to modulate