Before I get to explaining the differences of each, I'll first explain the one common thing all of them have - the ice type. Ice is probably the most pathetic defensive typing of all and even with Kyurem, that is true. It is weak to stealth rock, which is everywhere in ou, its speed tier isn't anything special, and it's weak to the spammed dragon moves in the tier
For regular Kyurem, it is mostly because it is just outclassed and harder to fit on teams. If you want a physical attacking dragon, why not use Black Kyurem? That 170 attack looks so tasty. It can carve out a niche for itself as a special attacker but once again, Latios exists. Kyurem has its advantages like not being weak to pursuit but it does not change the fact that it's still much slower. That speed tier is just not that good. The most recommended set for it on smogon is the subroost set with the self titled moves along with ice beam and freeze dry
For Black Kyurem, it is the lack of a good ice type physical move. Freeze shock is not an ideal move to use in any circumstances due to it needing a turn to charge up. Due to its speed, Black Kyurem often just gets one kill before being revenge killed. It does boast the strongest outrage, and to quote FSG, "it out Haxorused Haxorus" in this role, but that's about it. It still has the same problems that all the Kyurem forms have
Lastly, White Kyurem, just a simple question. Would anything in ou want to take a choice specs draco meteor that comes off of a 170 special attack stat and has a stab ice move that coms off that same special attack stat? If your answer to that is yes, then you need to check the ou list. If your answer is no, then there you have it
