I just set my ecobee to “auto”; the only possible inconvenience is the required gap between heating setpoint and cooling setpoint so it doesn’t constantly flap back and forth as a result over overshooting the setpoint. I have the gap set to 3 degrees and tend to shift it up and down seasonally (e.g. heating setpoint at 72 and cooling setpoint at 75 in the winter, heating 69 and cooling 72 in the summer)

Insolation and air infiltration based on wind conditions likely affect loss as much as outside temp, and the thermostat doesn’t know that (it could presumably learn to relate time of day, cloud cover, day of year (solar angle), wind conditions with that - but it would take 2 years to learn and its response would be opaque - most homeowners want bang-bang control since they understand it.