Would questions about Fury Road be on-topic here, or would they be better suited for the Movie Exchange?
Post apocalyptic fiction is an established sub-genre of science fiction. As such, there's no reason not to discuss the movie here on the site.
However, being a movie, it is also on-topic in Movies.SE. A common guideline here is that questions about plot and characters, and especially questions about the SFnal elements in the film, are warmly welcomed here, while questions about production, visual effects and the process of making movies is more fitting over there, but it's a soft guideline, and usually not a problem.
Just remember not to cross-post a question on both sites. That's discouraged.