Given the size of their staff, the rate they're pushing stuff out the door, and their desire for ultra secrecy (which would kill off almost all, if not all, playtesting outside the design studio), I don't think they're able to really do much playtesting. Some of the rules questions that pop up when codices are released do make it seem like no one really played the army much, because they'd spot those issues and fix them in the book if they had.