MF ult is completely fine
What is not fine is the E+thunderlord+Q combo in the face wich deals like 1/2 of your HP bar alone and there's barely anything you can do about it (about the E+thunderlord at least), all of this while she's faster than you and you can hardly retaliate.
The last nerf didn't even nerf her at all because she maxes E anyways and thunderlord + precision buff were huge buffs to her as a whole, the only thing they need to do is force her to get a ferocity keystone instead of thunderlord because she's silly with it, but MF with Fervor+Q max is completely fine because she has counterplay instead of Thunderlord + E max and doesn't auto-win trades just by pressing E.
http://champion.gg/champion/MissFortune/ADC you can see how the winrate went up DESPITE her being nerfed (W gives less AS, E slows way less), and as you can understand this is because of the cunning tree buffs.
So don't ruin an awesome and super fun ultimate just because you couldn't see that the problematic part of her kit is the E max + thunderlord interaction, and not one of the most counterable skills in the game.