After his transfer to Dortmund Hummels epitomized what I understand under the term "reliable, modern CB" for 2 years straight. It was then his partnership with Subotic, whom he slowly but surely began to outshine and sort of replace in his job as the defensive leader really was at its best level. Mistakes he made were so scarce I couldn't recall a significant blunder during that period if I tried - and even for the very best an occasional hiccup is quite common.
So Hummels only started to become quite a bit more unreliable after fully acclimating in Dortmund, and I really believe that has a lot to do with him getting more and more integrated into the team's offensive build up. Talent-wise, no other German CB (and there are a couple very good ones) comes close to Mats - only oftentimes his skills and efforts as "the first playmaker", if you will, come at the cost of less compact/organized last line of defense. His attacks often remind me of Lucio, who was also an absolute class act in defense, but much more aggressive about his goal-scoring ambitions. Compared to him (again: I loved Lucio back in his Bundesliga day) Hummels is really calm and concerned whenever he takes the reigns and tries to open up the game.
Stuff like the crippled pass against Real in the first leg are still an absolute rarity. He hasn't had a lucky year concerning his health, too. Little injuries often kept him from regaining full fitness and strength. I don't think the current season is the best showcase of his abilities, even though he still had plenty of great matches (The BVB-Real matches were particularly impressive; minus the blunder).
Badstuber only recently surpassed Hummels as a rock-solid CB. Prior to his injury (even earlier than that in the national team) he developed into a true defensive leader. That was mostly to his immaculate defending, though, less about his skills as a footballer. He does strike one as rather old-fashioned and a little stiff here and there. Nothing to be upset about as a supporter, since he's proven time and time again he'll get the job done.
You can't compare the Badstuber of now with the guy who didn't even play on the right position most times back when van Gaal was in charge. Pitting him against fast dribblers out on the sides was pure desperation. The team was lacking options. I believe many critical opinions are still influenced by some horrendous performances back in 2010.
It's much harder for guys like him to impress, simply because of their no-bullshit attitude. Negative impressions, however, are quite easy to leave behind and a real bitch to erase from people's minds - especially for defenders. Badstuber needed lots of picture perfect matches to finally gain acceptance; and to this day many of the more vocal supporters of Hummels, Höwedes, Merte (yeah, there are a few

, Boateng and co. just can't quite see "it". It seems Mats Hummels has a much bigger lobby within the ranks of the fans.
It's unfortunate that, yet again, this duo of fantastic CB's won't be able to step up their game in the national team any time soon. Combine them and you absolutely SHOULD have the basis for a world-class defense; only it seems that whenever Löw calls at least one of the two misses the trip. One day ... hopefully.
First and foremost Badstuber needs to heal - for real this time. The same knee-injury back to back cost Mathias Sammer the last couple of years of his great career. But that was over a decade ago and he was past the thirty-year-mark; so I'm not really trying to imply Badstuber might never come back. But it'll take time for him to regain confidence and strength until he can play at the highest level again.
(Jerome Boateng has also developed a lot this past year. He has become far more than just a placeholder for Holger or Mats. He's physically the most impressive of the three and not prone to blackouts anymore, it seems. And this guy is a real alternative for the wings.)