What I don't like from Hummels is that he suffers from concentration lapses too often and his positioning is far from world class IMO. He's no leader of the defence either which is another important attribute that can contribute to your importance to a defence (e.g. Thiago Silva for Brazil). For a defender to be good IMO, it's all about the brain rather than just physical characteristics. It helps if they have all of these attributes though, like say Thiago Silva and hence I regard him as perhaps the only true world class CB there is off the top of my head. As for Kompany, the guy was world class in 11/12. He is still riding off that season though and hasn't hit that level of dominance again. Well overrated now.
He is a very good tackler though, and scores a lot of goals for a defender. Which is why I think he's overrated, as it seems too many people judge defenders on their goalscoring ability and ability to make Hollywood tackles.
Ginter has intelligence well beyond his years. Will be world class no doubt and I wouldn't put Hummels in that bracket.
IMO there are too many players that are overrated and overhyped by the media and people without the intelligence to analyse football to any extent. Take David Luiz in the early stages of the World Cup for example.