Cresselia is a great addition to the RU Metagame. It before was a prominent member of RU when it was here many moons ago, for those that remember. Though a pokemon being prominent and successful shouldn't automatically deem it broken and banned. Cress has its place in RU, even if it slightly slows our fast paced, offensive metagame we love.
Cresselia has a wide range of moves, splendid stats, and the capacity to perform a couple of roles. Even with that don't imagine Cresselia as an iron curtain which can't be penetrated. With RU being an offensive metagame and Cresselia being a strong defesive pokemon this leads to interesting confrontations. Cresselia is able to check and wall a numerous amount of pokemon but at the same time many pokemon are checks to her. RU is occupied with various strong Dark, Bug, and strong offensive pokemon that can break through Cress. Common critics of the counters though are they are frail, but Cress itself is no power house. With only 75 Special Attack and it commonly only running Psychic and maybe a Hidden Power, Cresselia can be disposed of by several checks to it and walled by some.
Overall I think Cresselia is a nice drop for RU. Much criticism stems from that RU is to offensive. With Cresselia it balances that out a bit. It's starts to show a rise in defensive, and regretfully(lol) semi-stall in RU, and shows that these play style might be viable here. Going off of Pocket's post Cresselia is a godsend for weather team. RU has its faction of weather teams, but with Cresselia we will see more. This all going towards making a more perfect metagame with multiple viable plays styles.