This is true. The major problem for any home automation setup will be the endpoint user interface. Most of the solutions I have played with are very expensive for home use.
What I have been thinking about for home use are some cheap modbus thermostats from china along with a decent rs485 adapter. Given the nice python modules (modbusTK) out there it would be very easy to put something together python and some sort of web interface.
If you are going BACnet over rs485 good luck finding a cheap solution that can speak the protocol.