Why no one like Vraska?

[Everforward]

Why is it that not many people run  {Vraska the Unseen} as far as threat control I don't see much better

[Wally]

For 5 mana, by the time you cast her the game is well under way.
Also a lot of people feel that the 5 drop spot is already quite full with thing like thragtusk etc.
As for being a threat, she really doesn't do too much. They either just don't attack her when you use her +1 ability. She gets weaker when you use her -3 so they can probably take her out with 1 hit.
Very rarely you ultimate a planeswalker, and hers seems a bit of a win more ability. I mean if they can't stop / deal with 3 1/1's by turn 8(?) and you haven't already won then something is wrong.

[Silent1236]

She's ok.  Id probably only run her in some kind of control solely for her -3. If you can cover her, you've got a {Vindicate} on a stick essentially.  However, delving past standard, just slap her and {Liliana of the Veil} together in a nice, albeit expensive, bundle named {Karn Liberated}.

She's mediocre and a tad mana intensive.  Her -3 seeming to be the only "good" ability. 

[Everforward]

Ah, makes since