His actions during the 13th Crusade were highly suspect. He also has super magical disappearing powers which have nothing to do with the Warp - although he does spend a lot of time there. And a couple of other clues that are no longer in print so who cares?
But it's all basically just fan theorizing, right?


Anyway, point is, 'Waaagh! makes things work' may have been true at some point, but it isn't anymore.
There still is... in mild form. Even today. Ork weapons have the Unreliable quality in the RPG's, except in the hands of an Ork.