Meh, it's a superhero role after all. You don't need a powerhouse. Look at Hemsworth, Cavill, Evans etc. Not great actors, but serviceable enough for their roles. Or look at Bale, A-tier actor but not the best performance in his Batman roles. It's all about casting the right person. Affleck is a lesser actor than Bale, but from the looks of it will do an even better job as Batman.
I just don't understand his logic? You don't like the casting because of her career so far? Fair enough (even though you should reserve judgement until you see it, but ohh well). But to freak out like he did and say it was one of the worst miscastings ever? When I think of a horrible decision it's something like her ruining the character as well as the movie, as it turns out that is not the case. And he even said that it could work (without seeing the movie), so that automatically does not make a horrible decision. A horrible decision is when from the moment you made the decision, everything after is downhill.