I have no problems with Zero at the Bone, it´s one of my all time favorites.
I think it´s extremely well-written and I think she was brave to write that story. The whole thing with Ennis being a hitman, is very complex and in my mind it just goes to show that the author doesn´t shy away from difficult topics. And she also have the characters deal with it in the story, it´s not something that she ever lets them forget. It´s always in the foreground of the story. Not ever after their reunion, is Jack able to forget. It´s very interesting and the hitman/victim relationship makes for a hell of an exciting backdrop to a pretty epic love story.