@ Master Win: Talk of Yanmega and Cresselia have both died down. In fact discussion in general has died down I think because people are waiting for an official forum and suspect tests to begin. And everything that can be said about these Pokemon has already been said.
I think most people will have the opinion that Yanmega is broken, and there are solid arguments for why it is broken. To save some time, it's basically because of Tinted Lens and it's great power output, thus preventing any sort of safe switch in.
Some people may find Cresselia broken. The argument here isn't as clear imo, because Cresselia is just a bulky Pokemon. CM variants lack power so many viable Pokemon can come in and phaze it away, or haze it, Taunt it, set up alongside it or just outright kill it. Most Dark types, Water types, and Psychic types can win against it one on one and I'm not talking about a gimmicky eviolite thing either. Alakazam, Gallade, Drapion, Honchkrow (with Taunt and Roost), Blastoise, Crawdaunt, Sharpedo, Ferroseed... hell even Weezing can stop it dead (if the enemy Cresselia carries Psychic then you should just forfeit in awe of their power). Munchlax of course, being the best special wall in the tier, can give Cresselia a head start of +6 then come in to pick on something its own size. Need I mention Quagsire? Or SubSeed? Or Trick? So CM Cresselia is just another bulky booster that can be slapped around by almost anything not weak to it. In fact, it is pretty ordinary as a Pokemon and doesn't possess anything to distinguish it from other wannabe CroCunes...
Now we get to the support sets and here things get interesting. The power of Cresselia to guarantee a dual screen set up for the team because of its bulk and it's access to instant healing as well as good support options in Thunder Wave and Lunar Dance really sets it apart. This set is definitely a pain because Cresselia itself is ridiculously hard to OHKO so it is inevitable that it will give your opponent some sort of advantage before going down. If it even does go down. I can see how Cresselia is bordering on broken in this role because it can come in easily and set up support for your opponent to take advantage of, while you can do little about it. Cresselia doesn't have infinite healing, Moonlight's PP is a measly 8, so it can be stalled out if need be and support variants aren't going to be doing much damage in the meantime so there are pros and cons to using it.
This really requires more investigation I think, because how much advantage does Cresselia really bring? Would a team with defensive Cresselia really fare better than defensive Claydol? The team with Claydol will pack Rapid Spin and Stealth Rock of course but Cresselia's team will feature an almost guaranteed screens set up. The two teams will likely have different strategies to capitalise on the different benefits that these two Pokemon bring. I honestly think Cresselia and Claydol are quite evenly matched and I'm actually finding Claydol the more useful Pokemon. They compete with each other for a similar teamslot but bring different skills to the table. I'd be more than happy to keep it like that. Nothing solid has been presented to suggest that Cresselia is breaking the tier into it and its counters. At least not yet.