I personally don't think Kragnos is good in Big Waaagh. He doesn't generate you Waaagh points, or gain any of the benefits. He's much better in one of the other subfactions (Ironjaws, Bonesplitters, Kruleboyz) with the intent of him being a mobile hammer piece that gives your guys 3D6 charge.
I also think Big Waaagh lends itself to less large monsters and more units as they generate more Waaagh points, quicker. The name of the game with Big Waaagh is play defensive until you hit 20-24 Waaagh points, then start shredding. More Waaagh points = more Krumpin'.
10x Brutes or 6x Pigs with Violent Fury in Big Waaagh do almost the same damage output as Kraggy with their 2/2 profile at 20 Waaagh points with -2 Rend for way less points.
My 2c anyway.