To actually pull it off as a finish against a skilled opponent requires a pretty sophisticated setup.
From the turtle, I can't get anywhere near it on my students. I have had a lot of success with it from the Half Guard. Of course, this presents the problem of being skilled enough in the Half Guard to try it without worrying about being crushed.
From standing, I only try the Kneebar if I have lost the initiative and my opponent has taken my back. Otherwise, I play a bit more conservatively.