This set is absolutely deadly if you get the chance to get a few Mana Charges in. With such a bloated special defense, Kaguya can switch in against Suwako, Kanako, Shikieiki, Sanae... the list goes on and on. She can even take unSTABed super effective Ice Beams, Blizzards, Fire Blasts etc, and even STABed ones from weaker special attackers easily, and they won't even 2HKO her. The insanity rises to a new level if you get support from other team members in the form of Light Screen or Reflect, or baton-passed Calm Minds. Mana Charge up about 2 times, 3 if you can, slipping Recovers in while necessary, then attempt a "sweep" with Leaf Blade and Psychic.
However, here's where the problem lies. Leaf Blade and Psychic is terrible coverage, losing to many dangerous types like Dark/Flying or Grass/Dark. If you're playing with DP mechanics, Kaguya can add Mud Bomb and Shadow Ball to her repertoire, but they are pathetic unSTABed and don't help with the coverage problem. While Kaguya can wall super-effective dark special moves well, she will ultimaty lose unless the opposition does not carry some form of recovery.
It's worth noting that Kaguya's physical defenses aren't that bad. With 252 defence EVs, she can take a good physical beating. She also has two extremely helpful resistances in the form of Ground and Fighting. However, having said that, she has a glaring 4x weakness in the form of Bug. Kaguya must exit immediately upon the entry of Wriggle or Yamame. There are also quite a few touhoumon who can learn SonicBoom (70 base power Bug) by TM, but you will rarely see it ingame.