specs kyurem is probably the most powerful pokemon in the metagame atm, barring maybe kyogre. it guarantees a kill if you can get it in safely and the opposing team lacks a blissey.
kyurem black pretty much sucks as much as a pokemon with 170 attack can suck. it's "ok", but kyurem-w completely outclasses it and all it can really do is lure in ferro and ohko with freeze shock (it can't even 2hko with sr+spikes and outrage). LO kyurem-b or lefties sub kyurem-b is probably better than cb kyurem-b just because it's so much harder to wall it (not to mention you can do cute things with sub freeze-shock) but it still really isn't that good because you still get hopelessly walled with stuff like steel arceus, and forry uses you as horrible set-up bait unless you use hidden power fire, which still isn't doing that much in the rain.