There's nothing subtle about 12 Years a Slave. There doesn't need to be.
There's a certain amount of "it couldn't possibly have been that bad" in our collective understanding of American slavery. It persists in things like the Lost Cause point of view for the Civil War, and the fact that we conveniently share our Founding Fathers' bling spot with regards to slavery's original ensconcement in the Constitution. There's a recognition of the inhumanity of slavery, but simultaneously an inability for us to really grasp just how awful, dehumanizing, and corruptive slavery was for our nation.
The best rebuttal for this kind of historical revisionism is the truth. That's what 12 Years a slave brings us, pure, unvarnished, and lacking any kind of justification. It lets the behavior of our forefathers carry the story. As difficult as it is to watch, it's a necessary movie.
It really was that bad.