The quality of dialogue seems to be proportionate to the quality of the actor. For example, Cillian Murphy and Tom Hardy are superb actors so get imaginative and sharp dialogue. Michael, however, got lines that would be edited out of the worst type of Gerrard Buttler film.
The portrayal of Mosley and Mitford as cartoon baddies didn't really help its credibility. I'm not sure what the ethics are of suggesting that Diana Mitford would sleep with people simply for various manipulative purposes but I could imagine her descendants wouldn't be thrilled (if she has any left alive). That said, both she and Mosely were vile and weren't sufficiently shunned in their lifetimes so maybe it's just karma.