There a few things you can do that can help.
1. Close vents in the rooms that are too cool or partially open them.
2. Use motorized vents, not sure if there are any that are homekit yet but there are a number for SmartThings system
3. Have a booster fan installed in the duct to the room where the problem is and it goes on by a separate thermostat.
I have been installing building automation systems for 32 years and those are the easiest things to do and least expensive. The design of home HVAC blown air are not balanced systems where they are designed with bigger ducts to certain rooms, automated dampers with fan units in each room.
In our house the hvac system is terrible. The house has been remodeled and the layout of the system is really bad. On the main floor I have a supply and return right next to each other in two spots. I have blocked off one supply and one return to help cross room circulation. Our kitchen which has west facing doors to the deck has one supply and no returns...
Our biggest problem is lack of proper flow volume. In the master bedroom upstairs we have two supplies and one return. When we moved in the return was "covered" by the clear duct shield I suppose to slow down rate of return?
What we have found is it is better to have one of the supplies blocked off so that air pushes out at a greater rate from the other supply.
I would love to install a booster fan in the main duct which is going to upstairs. The access I have is rectangular stack. In this situation what sort of fan would you recommend? This would be installed in the basement. There is no access to any of that main stack until the attic. Like I said the hvac and more... is a Homer special. This is my first house and you sort of learn and figure out and have many aha moments after purchase.
Going back to the OP. I have a single system and I opted in for a electrical supply install and rebate. I paid $150 for ecobee with install. The installer did have to install some gizmo to provide power to the thermostat.
The way we have it setup is at night we tell it to read the temp reading from the master bedroom.
When the AC is on we have the fan to run constantly, when away the fan runs for 10 minutes per hour.
The iOS app for iphone works good, for iPad it has more features / graph trends. You can also access this info via online portal.
Overall i'm really happy with this smart thermostat. It has been solid for several months now.
Due to our bad design this does not always work but the thermostat is smart where it factors in weather etc and if you set 71* for 4:30 PM a normal programmable thermostat would turn system on at 4:30. This ecobee will run prior so that at 4:30 PM the temp IS at 71* as requested.
For myself this is first Smart home item as well.