He's not as good as he's being made out to be. Sure, he's tall, aggressive, pacey, good in the air, and comfortable on the ball (though not necessarily great), but he has one glaring weakness: he ball-watches a ton, which makes playing past him relatively easily. He also gets drawn out of position relatively easily due to his aggressive nature. Interestingly, many ball-playing CBs have these weaknesses, but they tend to make up for it with their much better ability on the ball (dribbling and/or passing e.g. Koundé) and/or their athleticism (pacey, very good in the air, and/or strong on the tackle e.g. Joe Gomez or John Stones).Sounds like a perfect CB. But why he would be that cheap and not hyped more than this?
Milenkovic can't show much of his ability on the ball due to Fiorentina setting up their back 3 to be conservative, and much of their attacking threat comes from open play via Ribery/Vlahovic/midfielders. Honestly, Fiorentina aren't set up to get the most out of Milenkovic; a more enterprising team might get more out of him with his supposedly good ability on the ball.