I find the most frustrating aspect of Cresselia is that when facing one, foreknowledge is of utmost importance. I've had several battles where an opposing Cresselia was their last stand and CMed to victory. If a rematch were to occur, I would have to remain extremely vigilant on my Cress "counter" or "check."
The most terrifying thing about Cresselia, I believe, is her versatility and ease of use in this tier. She is great as a dual screener, status inducer, and has trick/lunar dance/etc. When facing a new Cresselia, you must quickly recognize if it's physical or specially defensive, or if it's built with speed to defeat some of its weaker counters. She doesn't have a glaring weakness, tho there are dark types that can threaten her. There is no doubt this is the closet pokemon we've encountered in UU to the defensive BL characteristic.
Umbreon is a decent check to Cresselia, and is actually a good parallel to her. Her curse/wish/heal bell/payback set is bulky enough to walk a good portion of the metagame and slowly ensure a sweep. However, there are some relevant differences. Levitate allows Cress to effectively switch in on the ever common EQ and prevent it from taking Spikes damage. Two 120 BP moves, Superpower and CC, can quickly shut down Umbreon, despite one or two Curses. In contrast, answers to Cresselia include, Nightslash, Pursuit, U-Turn, non-stab Megahorn, and a variety of SE special attacks that are watered down quickly from CM.
Cresselia has a decent movepool and it's important not to theorymon 6 moveslot terrors. There are definitely answers to Cresselia. But I believe these answers are shaky and cannot easily and effectively exterminate Cresselia without foreknowledge. Once the EV spread and moveset are revealed, Cresselia becomes a tangible target. Perhaps players are using her poorly, like a sponge the way Uxie can be played. I've countlessly been swept by last standing Cress and that is where her strength lies.
I believe she's the closest thing to defensive uber we've seen.
Oh and Moltres + Spikes is broken.