It's a good buzzword to sell someone on the public based on talent rather than pedigree. It does get overused.
I don't know how you could argue Snyder isn't visionary, though. Visionary doesn't mean "greatest director ever", but Snyder is legitimately creative in a very visual sense. He storyboards all his stuff by himself, from the ground up. He has a great understanding of the process and what imagery he wants to capture.
If you contend that the adjective is being used to state that said director is ahead of his time, then I can see the issue. This is a Hollywood buzzword to describe a visually compelling, usually young director - not to state that he is shaking the pillars of Hollywood with his talent.