The boys are ta'veren, so the laws of reality themselves are on their side. That's some fairly significant obstacle. As for the wondergirls, when has the Shadow ever tried to have them killed? Since the plot to have them sent to Seanchan (alive) in The Great Hunt, there's been nothing of significance attempted against them except for Moghedien's spur-of-the-moment attack on Nynaeve, and a few reactionary moves like the Black Ajah defending themselves when ambushed by Elayne.