YY is not currently a meta deck, not in terms of how far you can go. It is currently the best midrange deck for both coop and a decent midrange deck for pvp (though really painful when it gets countered, which is a lot).
What does go far in co-op these days is:
YY can go beyond its normal limits with support using not just assassin but also time and/or Healing dice, and especially on the Pantheon field, and recently even better if the YY player is skilled and can manage a couple of Crack dice while still getting 7 pip YYs on power squares in harmony. But that's a tall order and out of reach of many players.
YY for the more average player does start reliably reaching 51-55 at dice class 9, and with the Club Violet speed field and assassin support it can sometimes go to wave 69, depending mostly on boss sequence luck. maybe a bit farther with ass+time help.