Wraiths (of both types) are fragile shock units that cause terror and deal massive AP damage. They absolutely need to be used in conjunction with fodder troops holding down the victims so the wraiths can charge in, do a ton of damage and hopefully cause a short rout through terror, getting in more free kills from chasing. They're hit&run units - big impact against their prefered targets, but vulnerable if caught in an extended grind. Hexwraiths in particular are great once you realise that you need to use 'em like light shock cav in Attila or Rome, except that unlike those they specialise in ruining heavily-armored troops. And frankly, units that do magic damage are exceedingly rare even amongst elites. In fact, I don't think any Empire, Dwarf or Greenskin unit does magical damage except for the Luminarc of Hysh. A few VC units (notably the wraiths themselves) do, as do bretonnian Grail Knighs.