KNX is strongest at the room, BACnet at the plant. Reading both gives rooms and the plant that serves them in one view.
What each was built for
KNX targets room and building control: lighting, blinds, heating and presence, with devices sharing state through group addresses on a bus. BACnet targets building management: chillers, air handlers and the controllers that run them.
How data is identified
A KNX group address such as 1/2/5 has no name of its own. The ETS project that designed the installation holds the names and datapoint types. A BACnet object carries its own type, properties and units, and can describe itself on the network.
Security posture
KNX IP is unsecured by default, and KNX Secure adds encryption and authentication where the installation supports it. Classic BACnet/IP is also unauthenticated, and BACnet/SC adds secure connections on devices that support it. In both cases, network segmentation matters.
Why buildings run both
Hotels, offices and campuses commonly put KNX in rooms and BACnet in plant rooms. The questions that matter, such as why a floor is overcooled or which rooms are empty but conditioned, need both datasets.