RCA connectors are infamous for impedance "bumps" in the signal chain in RF applications. Since they are not 75-ohm, they can set up signal reflections along the transmission line. Those reflections become a standing wave and that can interfere with the wanted signal. BNC, TNC, N, C, etc. are constant-impedance connectors, when properly selected and installed. Not just any BNC will do, as there are both 50-ohm and 75-ohm BNC connectors.
Why so few devices with BNC's??
It's an ongoing amazement to me how many manufacturers use RCA's for 75 ohm digital connections. Is this really to just save a couple bucks? Lower end McIntosh stuff has RCA's as does most Japanese gear regardless of price. It's not like BNC's are really so exotic, and 75 ohm cables are readily available. In fact, the general lack of inputs is an annoyance. Not everybody wants to use USB or Toslink. Rant over. 😠 Thanks for reading.
[Please, this is NOT a thread to list all the exceptions.]