Just flew this itinerary with a 15 hour layover on the way back in Belgrade. Flights and Belgrade airport are perfectly fine. I can walk the whole airport in 5 min. Had an empty seat next to me on 3/4 flights. I went into town and saw some attractions, and while not much to do (at least at nightime), I felt very safe. Happy to answer any questions.
p.s. they also give you a hotel if you have a long layover but I found out about it too late