The room is just a JSON object, too (or a "path" down inside one), so anyone you want can be in the room. If only two people are listening to that object, it would look like private messaging. If everyone were listening to a room, it would appear "public."

The disadvantage is that you have to build it. It's easy, so I'd say that's really an advantage because you will deeply understand it. NoSQL data and websockets are really important technologies and Firebase is free, including hosting, until you hit some pretty high usage limits.