Everything "sucks" (has problems) the ways in which X11 "suck" are very well documented and understood, and there are few, if no surprises.
Sure, major version revisions that break the previous API exist. But in my experience there's a new one every couple of years, like branches on a tree, the main trunk will never change.
Sure, major version revisions that break the previous API exist. But in my experience there's a new one every couple of years, like branches on a tree, the main trunk will never change.