We could test them, but I think "no ban" would be an overwhelming majority.
Mega Diancie
Ok, it's awesome, has the best spread move, but it has some things going against it. First, the turn in which it mega transforms is very dangerous. It's a turn in which it is both slow and "frail", having 50 speed on regular forme and then 50/110/110 defenses, which ain't precisely low but you are getting some damage there. You kinda need to mega evo + Protect., which is predictable and opens hole for opportunity.
It's weak to spread moves Earthquake, which is everywhere, Muddy Water, which sees some use (and could see more tbh), and Surf (why are you using Surf). Earth Power from Heatran, that's used a lot. There's Bullet Punches which simply obliterate it. To use Mega Diancie, you need your team to clean up first. And cleaning up isn't as easy as it seems usually. Ok, give a team that doesn't have any of those and mega diancie can basically win the battle spamming its signature move, but that's true of a lot of Pokémon.
Kyurem-B
Isn't this kinda a jack of all, master of none? It doesn't have any good physical STAB. Dragon Claw sucks. The CB set is decent, but it's kinda ruined by the everpresent Will-o-Wisp (and it can't OHKO Rotom) and intimidate spam everywhere. Scarf set, ok, it's good, you get that lando KO, but it's still not that good, as you may get locked into Ice Beam, or Fusion Bolt, and then have something throw a couple of heavy attacks (or once again burn) at you. It needs Fusion Bolt to get past Azumarill. Mega Diancie destroys it. I love the Sub set, but you need to force switches and some times eliminate some threats to use it decently.
I don't see the need for testing either, they don't overcentralise the metagame nor break usage records so far.
PD: Testing Zekrom was semifacetious, I guess it wasn't clear.