The RCC membership doesn't get you into everything that *G does, e.g., certain *A contract lounges. An example: While Lufthansa's lounge at BOS was under renovation, they had a deal to allow *G customers (but not RCC members) to use the AF lounge.
I also had trouble getting into the TP lounge at LIS. In Europe, it's mostly just accepted that *G = lounge access, so the agents don't always know what to do with the paid memberships.