* You dish out Mortal Wounds like they were candy. For a good example, there's the Abhorrant Ghoul King on Royal Terrorgheist. With the right combination, he can get Always Strikes First and deal 20+ MORTAL WOUNDS from his Terrorgheist's Gaping Jaw!
* Their insane nature opens up many narrative ideas for your battles, as well as conversion opportunities.
They'll only run if they're pounded into the ground and even then there's a chance they'll stand.