If you think about it normally, it's an Overpowered card, but since it requires the maintenance of Zombie Worldto fully demonstrate its effect, it doesn't matter if it's Mystical Space Typhoonor anything, so it just ends up with a random back destruction
Like those, it was banned to avoid the perceived-barrier-to-entry, but was strong enough to be a candidate for banning even without the optics especially in long games where the card draw yields inevitability over time
The real power is in his second ability, a zero loyalty cost effect that mills ten cards from any player
Steel Overseer is the most straightforward and accessible, but Metastatic Evangel, Felidar Retreat, and Virtue of Loyalty are also valuable
Half the treasure is the glory of finding it
In blue, you gain access to repeating tappers such as Narcolepsy and Ghostly Touch