I wouldn't really compare him to Moises Caicedo. Caicedo's actually an unknown entity to the European football fanbase, but Milenkovic was way beyond that. Anyone who follows the Serie A even casually would know about him (unlike, say, Noah Katterbach in the Bundesliga for casual Bundesliga followers). There's a reason for that, too: Milenkovic has been established in Fiorentina's first team for the past 3 seasons, and to do as a defender from the age of 19/20 is quite impressive.
The reason he's not more hyped up is because of the following:
- Fiorentina have been generally poor and don't play in Europe
- Milenkovic isn't so expressive on the ball due to Fiorentina's system
Despite that, however, I agree with you over how overhyped he is starting to be. He has all the tools to be a great defender, but at the moment, he hasn't been as imposing or as dominant as Konaté or as extravagant as Upamecano or Koundé.