"Dark Nuns" is a slow burn, demonic horror, thriller.
What works well in this film is the notion of taking on, what increasingly, looks like, insurmountable odds. This is no ordinary possession, its a Duke of hell, one of Satan's most powerful generals. You get a real sense of the threat and dread presented by this powerful, timeless, evil spirit though its dialogue and its manipulation of people and the environment. Its not to be trifled with.
The concept of two nuns exorcising the heavy duty supernatural baddie as opposed to a priest, is novel too. Its cleverly handled, turning it into a macabre, almost maternal, drama.
What does not work so well the over long length, populated with drawn out asides and sub plots.These unnecessary distractions, water down the films key focus.
In summary, "Dark Nuns" would have worked considerably better if it had adhered more closely to its core story and not wandered off script, so to speak. Worth a look but still less than it could have been.