I believe the protocol that Crestron runs on, as well as Control4 is CA$H. it can cost $200 just to change the function of a light switch.
Just kidding, but what the others have said is correct, it is pretty stable and reliable system in general, often used in commercial spaces for their AV integration. Think high end conference room. also in expensive houses
Control4 uses Zigbee, and I am not sure of the actual protocol Crestron uses, but it is always installed by a dealer who is certified and keeps a tight rein on their programming software