I didn't have a "full" setup, just a Nest and a few rooms on Hue; I ended up leaving it all behind— the realtor actually had the Nest right on the listing like it was a major selling point, and our new place had an older boiler anyway, so it wasn't clear it would be a good fit.

It was fairly easy to just factory reset everything and walk away, but yeah if there was an actual PC/RPi and a user-managed software stack in the picture, I could picture it being a real headache to sort out.