Ashiok gets better every time I reread her.
Against control, she's a slightly easier to deal with Drownyard that'll cast a win condition from your opponent's deck if left unchecked.
Against aggro, she either 'gains' you 5-7 life or,,, wins you the game.
Against midrange, GG. She just crushes midrange strategies. Cast one of your opponent's creatures -- even if it's something to just chump with -- and you're in an extremely favorable position. As in... you pretty much win. She just gets loyalty too fast for a cheap walker. If you don't dreadbore her, you kind of lose.
Follow her up with supreme verdict, get her back up to 4-5 loyalty from the previous turn's attack, then start casting my opponent's creatures for free? Okay. Yeah.
She's hard to evaluate... but that's a lot of potential from such a cheap walker.