The Cantarranas pitch is right in the middle of the UCM campus. It's also where another team, "the International People of Madrid RL" are basing themselves. I sometimes take my wife all the way to work in Aravaca and the bus goes by the side of the campo. It's a bog-standard grass rugby pitch really, which will be like playing on concrete in the summer.
However, out where the A-6 meets the M-30, is Parque Deportivo Puerta de Hierro. There's a 3G rugby pitch with bleachers that probably seat around 250. Tennis, padel, swimming, golf. The problem is that I think this is more tightly controlled by the RU federation in Madrid. The other issue is that it has limited public transport, just two buses stop nearby that go in and out of Moncloa. However, it's a much better setup than Cantarranas.