it doesn't work on the majority of systems i have to work with, which is servers that don't have the necessary tools installed.
it's also very susceptible to latency and most applications don't handle slow connections in a usable manner. (they are designed with the expectation that the gui always responds instantly)
to get something of a tmux/screen like experience, xpra is available, which is an awesome piece of work, but it doesn't help with the latency. even over just local wifi i have some applications become unusable over xpra when they work ok over plain remote X.
the problem is not necessarily X but in part GUI in general. i can't click the mouse anywhere until the respective UI item is visible, so i have to wait for that.
on a commandline on the other hand in most cases i can keep typing even on extremely slow connections because i can anticipate what will happen and i know what keys are appropriate to type next.
using mosh i even get something like editable typeahead which is a marvel and very hard to imagine on X.
greetings, eMBee.