Only one of Kyurems forms is OU, and that is Kyurem Black.
The reason why it's OU:
It's movepool. It Kyurem itself has a great special movepool, hence why Kyurem White is in Ubers. Kyurems physical movepool is meh, with its main physical moves as Iron Head and Outrage. It gets other physical moves as well, but they're okay. If Kyurem gets more and powerful physical moves, then it would be Uber material.
Here's Kyurem's pokedex page to see its moves and why it barely has any good physical moves.
Hope I helped!
Source: Above link