[Taqwus]

In the case of a Vastness, mobility plus the lack of need for a magical commander. The Vastness is capable of moving 10 provinces per turn or even underwater, but if it's a unit can only do so under the command of a commander with similar mobility and magical leadership. Such mobility tends to be rare (Horrors, if memory serves, have similar amount, but the only guaranteed way to get, say, a Doom Horror that fights for you instead of killing everybody is a Wish), and most magical commanders aren't quite as buff. If you lost all magical commanders when attacking with a non-commander Vastness, you'd lose the Vastness too.