X works seamlessly, it's a client/server arch.
Even if you run emulators, by exporting $DISPLAY to your host (and by running xhost +$EMULATOR_IP) you can run, for example, tools for older Unixen.
X's IPC is very stable (which is half the point of X); the instance on shared libraries has more to do with it being a memory hog on the kinds of systems that existed the first couple decades of it's life.
This. If you rum SIMH to emulate a Vax (and some other Unix), most Unix programs requiring X can be perfectly forwarded to use the host's X server. Or some Xephyr server.
Except for SGI, but I am not that sure.