It's a 1.5 second stun ult that she has to aim in the correct direction when Zed pops up directly on top of her. (In other words, predict which way will work without waiting to move away from Zed's emergence location because she will die if she does)
If she ults before Zed ults, she runs the same risk of trying to Power Chord Zed; the projectile gets ignored.
Meanwhile, Nami can bubble, Janna can tornado, Lulu can polymorph, Thresh can hook and flay, Blitz can grab and knockup, any number of Supports have non-ultimate hard cc with the same duration or better.
As said before, her damage is getting nerfed, which was already lower than the damage from Nami's W at level 9. If she were to somehow survive long enough to get the auto off, it will be the same base damage as Nami's W at level 9 after the nerf. (Though Nami has more AD) her heal and shield has always collectively been lower than Nami's W at level 9, (and the heal effect still less than Nami's at level 13) her speed boost is not by any means an escape, and her ultimate is an ultimate, and furthermore has a momentary but unbelievably annoying channeling time. (The kind that an assassin like Kassadin or Zed can legitimately finish you off in, assuming death recap can be trusted)
Also, her abilities have a .5 second static cooldown, so she can't cast them simultaneously.