His Preacher and his Punisher are both great - but he seems to delight in controversy for it's own sake. Like the class clown who won't shut up, just to give an example. Mark Millar has a similar rep, stirring the pot for it's own sake( re. his work on Authority); taking pleasure in being a bad boy in the medium.
Ennis is very good at what he does, but at times he can start to wear thin. Punisher was good for the first few arcs, but we never really cover any new ground. Frank punishes criminals in horrible and darkly humorous ways ad infinitum. Similarly, Ennis's absolute disdain of most any superheroes will prevent him from ever really having a solid spot in the mainstream.