Keeping Kefka alive a whole turn cycle to use his ability again might be tough, but at least in black there's a fair few protection spells
Essentially, creatures are always fair game, as are artifacts and enchantments (though theres less removal for those card types at more casual tables)
This card basically gives you an additional 3 turns every pass around the table
Meanwhile, Vannifar can add face-down creatures to the board by cloaking them from your hand and if youre not interested in doing that, you can also buff all of your face-down creatures ever combat
Conclusion There you have it, my friends, everything you need to know about the serialized Doctor Who MTG cards
The flavor of legendary creatures exploiting weaker, less memorable ones is on point and powerful