Her +2 is fine with me. Sure, you have to discard a card to draw a card, but with Madness making a return in Shadows, this is fine. She’s a fine enabler for that. Her -2 is a decent way to deal with problems, albeit they have to be tapped if you’re exiling an artifact or creature. But she can also exile an enchantment with this ability, too. Alone, those are two decent ability for a 4-mana planeswalker, and she starts with 4 loyalty. Not bad so far.
Her ultimate ability is pretty sweet. For 8 loyalty, she can search out any artifact or creature from your deck and put it right into play. It gains haste, but has to return to your hand at end of turn. If you have a card that can just win the game in your deck, this is pretty sweet. Having to return to your hand isn’t really that big a downside if you pick the right card.
For most people, this ability means Blightsteel Colossus in Modern or Ulamog, the Ceaseless Hunger in Standard. But really, her ultimate ability is based on whatever the current game state is. If your opponent is vulnerable and you need a linchpin, this is how to get it. She also would work fine alongside Chandra, Flamecaller from Oath of the Gatewatch. So I can’t see her not doing some work in Standard, even if you only ever use her first two abilities in a game. 4 mana to exile two problem things is fine, especially when one or both of those things are tapped Eldrazi.
I think she’ll really shine in EDH, personally. Sadly, she can’t be a badass Commander the way Nahiri, the Lithomancer is. But Nahiri #1 could only really work in a deck based completely around equipment, which actually has turned out to be pretty good in EDH. The Harbinger can fit into a lot of decks. I can’t think of a Boros (Red/White) deck that wouldn’t want to have her.
In Modern, Ajani Vengeant does a lot for only 4 mana, so she’s competing with him for playability in that format. And Vengeant doesn’t really see a lot of play in Modern anymore - which is sad. That being said, she could be an answer for Eldrazi in Modern, but at that point, much of the damage is already done as whatever you target has to already be tapped. I think she’s best suited for Standard.
While I prefer Nahiri the Lithomancer flavor-wise, I think that the Harbinger could be quite a playable planeswalker. She fits really well into the themes of the new Innistrad block, too. I’m excited to be able to play with her.