Yea... Warrax is my favourite too.
I think wraiths are better than shadow demons. They can drain very good... so you kill weakest units first, and get very much life. Then, you can kill even great drake with them. Shadow demons can only dream about this.
So.. you can go with wraith even to most of nodes. You also get any neutral city very easily... and you will raise army of undead, which can be defense for that city... for no cost! That's why i prefer wraiths.