I think that the major issue Is that they removed the one thing that made MF so strong before, her w, which most people overlooked and didn't even notice. Before on MF her W allowed her to get AS and Full stacks of her passive and grievous wounds. I think that riot really crippled her damage when they removed this, mind you regaining strut is really nice but its to hard to get the passive stacks on her w up quick enough to deal any real dmg.
This is her ult for anyone who doesn't know the stats on it.
Miss Fortune channels a flurry of bullets into a cone in front of her, dealing large amounts of damage to enemies.
Miss Fortune channels a barrage of bullets into a cone in front of her for 2 seconds, dealing 50/75/125 (+20% Ability Power) physical damage and applying a stack of Impure Shots with each bullet (0 additional magic damage for each stack already applied to the target)(0 represents the amount ur impure shots does at the time).
Passive: The stack limit for Impure Shots is increased by 1/2/3.
So before you would pop W then enter bullet time and have them being hit by full stacks for the entire duration of the ult and applying grievous wounds so they wouldn't be able to heal up immediately.
It was such a large part of why she was good in AOE team comps, but with the removal of her W active it her ULT now does whimsy dmg and is now only used to wave clear and deal minimal dmg until mid/late game with items armor pen items. (even then its very underwhelming)