i was going through my WH 'dex today and thought "hey it would be cool to make an entire army of WH adversaries to play in a big event."
now if you look at the rules for that army, you'll see that there's some room there for a bit of abuse because of the number of minimum-size troop selections available (infinite) and their ability to infiltrate.
i'm thinking it's probably one of those 'call ahead of time' armies where you'd have to make sure it's legal per the rogue trader you want to play it in.