Make the clients listen to their own cards something like:
room.listen("cards/player1/:cards");
room.listen("cards/player2/:cards");
etc.

Also on a Unity based game with colyseus, is there any source to understand how to process every data that the server sends, it's a hell having to cast every parameter.

Re: [Unity3D] OnMessage issue with JSON object (SOLVED)

Referring to this post, @endel uses msgpack-unity3d to read the data sent by the server, I can't find any examples and it was a tad tedious to read some parameters sent from my js server.

I'll explain more in a bit.