Someone counted the number of minutes Bana was on screen: under 5. That's about as cameo as you can get for the main villain of a movie.
Ya 5 minutes pretty much is a cameo which is strange for the main antagonist. I thought he did a good job with what he had but his talent was largely wasted. I hope in the deleted scenes he has more scenes.
Yeah, 5ish minutes is a cameo in my book but I'm totally fine with it. I wish there was a bit more back story on Nero and what drove him to where he was in the movie, but I still love the flick.