Before Emet-Selch, I would have scoffed away the theory that Urianger is an Ascian. But now? I'm not sure what he is. If the theory of Urianger being an Ascian held any color of truth, I would think that Emet-Selch would have blown the whistle on that by the end of 5.0. Emet threw all caution to the wind when the Warrior of Darkness and company confronted him before the final days of Amaurot, I, myself, would like to think that he'd have straight-up called out Urianger on the spot during his catharsis. And if Urianger were, indeed, an Ascian, would he have so easily been struck down during the cutscene, when Emet shot him with magic? Maybe not, unless he's a very cunning actor and can feign weakness.
Long story short, I think the possibility of Urianger being an Ascian is probable, but I still have my doubts. It could be his own complex individuality that makes him who he is.