I can add to summarize it all:Thank you everyone for sharing. I synthesize:
For unbalanced audio BNC has the following advantages over RCA: locking, higher reliability
For unbalanced audio connector impedance does not need to match cable impedance. This only matters for higher frequency signals.
Almost any connector type is better then RCA, because RCA is the worst connector type. The only good side - it is cheap (and popular in audio).
I see these for USD25 a pair, so not a cheap option.
Cheap is the main reason for the RCA