I always felt Miller was very talented. You just need to watch We Need To Talk About Kevin and contrast that to their roles in Perks of Being a Wallflower or as Flash to see the range they have. I think people take issue more with the writing of the character and dump those issues on Miller.
On the other hand, you're right that Miller is clearly somebody with deep problems that probably needs serious help rather than headlining a major franchise. Considering that WB apparently considered shelving the movie over their behavior, I doubt they want to roll the dice on them again.