Although X11 is flexible, powerful and time proven, it has a few weaknesses.
First, audio data is not transmitted by X11 messages. This “oversight” was most likely due to the fact that most Unix workstations did not have any mentionable audio systems, and most X11 client programs are either engineering or scientific, making audio output a very low priority.
Second, the X11 protocol is not exactly a secure networking protocol. It is not recommended in a network that may contain untrusted hosts.
Last, but not least, the X11 protocol requires a bit of networking bandwidth. When it was designed, its only application was across computers of the same LAN. As a result, X11 works slowly over a slower Internet connection.