I liked Shame, but I didn't think it was a great movie. It was ambitious and I liked the way some things were directed, but it was atmospheric and a bit short on drama or convention drama. Michael Fassbender was great in it and it had a lot of long shots/scenes that allowed him to stretch as actor. It was more of a character study than a robust story.
After hearing great reviews for Once a Slave, I could see him really doing well with this material that is obviously loaded with drama.