A lot of Pokemon have sets that are deemed 'uncounterable'. As a matter of fact, most good wallbreakers in OU should rightfully be 'uncounterable'. (EX: Garchomp, Terrakion, Hydreigon). Hydreigon alone invalidates the argument that a strong wallbreaker that can defeat almost every Pokemon in the game switching in with its myriad of options is the least bit overpowering. Hydreigon is let down by its speed and common weaknesses, which allows a fair number of Pokemon in OU to check it. Kyurem-B suffers from the same issue. For all of its defenses, it is still quite easily swept past by many offensive Pokemon in OU.
The movepool argument has some merit. Its only safe physical attacks to use are Dragon Claw and Fusion Bolt and neither moves can defeat his checks on the switch. STAB Ice Beam and Earth Power coming from 120 base Sp. Attack are very respectable, and allow Kyurem to remedy some, but not all of the holes in its offensive game. Unlike OU's tried and true powerhouses, Kyurem-B has no viable boosting moves whatsoever, meaning that no matter what, you are stuck with the same offensive stats that many other Pokemon have the option to boost with the likes of Dragon Dance, Swords Dance, and Quiver Dance. With its unimpressive speed and exploitable weaknesses to overcome, this is definitely a problem.
Also, with its only standout traits being supreme bulk (shared with Kyurem, mind you) and a gigantic Attack stat, don't you find it strange that an allegedly bannable Pokemon is stuck covering many contingencies (SR + Spikes weakness, lackluster physical movepool forcing it to go mixed, a large number of checks) that other offensive simply don't have? Kyurem-B is definitely competent in OU at what is does, but I think people here are trying to force an issue that simply doesn't exist. Kyurem-B is a victim of the 'newcomer' syndrome and nothing more - if you're going to take issue with him, you'll first have to address the old dogs who are quite resoundingly better than he is.