Kyurem-B is far from broken. I usually play on the PO server. You can joke all you want about how it's not as good as the competition on Smogon, but it's solid. On this server, Kyurem-B has been unbanned for 2 months or so. At first there was a lot of outcry over it, but once it was released, all the complaints disappeared.
The reason for this is it's lack of Dragon Dance, disappointing speed, weakness to priority at a time when Scizor is the number 1 Pokemon used in OU, (maybe 2). Its really just a worse Haxorus -- do 23 extra Attack points and Fusion Bolt make that much difference? Are they worth a weakness to Bullet Punch, Stealth Rock, and a lack of Dragon Dance? On PO, it is ranked 92nd in the usage stats, right behind OU powerhouses like Zoroark and Whimsicott -- remember it has been legal for months there, so people are able understand its power. I used it for many games, CB, Scarf, SubRoost, and Sub + 3 Attacks, and every time I did, I found myself wishing I had brought Dragonite instead.
It's a threat you should worry about, and yeah it does wreck stall -- but Genesect, Keldeo, Thundurus-T, and Sheer Force Landorus have already done that, and it's not much threat to offense, so what's the point? Hydregion has no counters either, and look how useful he is. What counters Specs Keldeo in Rain? Not much. Is he broken?
^Edit: Deo-N is broken because he has base 150 speed, Rayquazza because of its greater effective bulk, given its typing. Give that to Kyurem-B, and I would ban him in a heartbeat lol