Eh? They had GREAT voice acting in FF:tSW. Maybe not the pinnacle the way some Disney flicks are, but they had solid voice talent. Ming-Na Wen, Alec Baldwin, Steve Buscemi, Donald Sutherland, James Woods, Ving Rhames... How can you really complain? Perhaps cheesy dialogue took them down more than they should, but...
And overall, it was a typical, "bubblegum" sci-fi plot. Not really different than any different than a Star Trek film or generic sci-fi-sans-real-aspiration. I think it failed primarily for marketing reasons, as it drove away people who thought it was "a movie of the game" and drove away the FF fans for NOT having anything to do with a game. If they'd just left it sans "FF" they could still have promoted to Square fans just by BEING Square and everyone knowing they did the animation, and appealed to the broader and much larger sci-fi moviegoers who just like random flicks and might be attracted to the impressive, realistic CG-ness.