I play a LOT of MF
Her ult is ok , great early/mid game but it falls off immensely. I'll make a short list of what I find to be her basic problems.
Her passive: While handy in early game lane phase, and great for positioning. It simply is broken far too easily. It should either be lowered and simply offer a flat move speed boost always, or possibly do something similar to Garen's passive where it isn't as broken as easily come later stages of the game.
Her q:Only a small issue and I don't expect it to be fixed. Most poke adcs have a long range poke ability that does not draw minion aggro. Now this wouldn't be that big of an issue, except her passive gets broken by poking the enemy. This is more an issue with the passive however.
Impure Shots: I miss season 2:( That being said, I realize why it's not as strong as it was back then. However I do feel it should be changed. I'm ok with the 5 stack system, but how the ult increases the stacks to 6/7/8 is problematic. The only way to get to that level of stacks is by ulting before the fight, and even then after the ult the stacks will fall off of most targets as you can only focus one target at a time after that, barring a Q bounce. It's simply unrealistic to get beyong 4-5 stacks at best, except against the tankiest of targets. I believe the W ratio could be mildly increased with each ult stack instead.
Her Ult: It's mixed damage, but heavily tilted in the physical damage range. Even if the enemy stacks armor, the magic damage portion is not felt heavily by the enemies. I realise it's not meant to destroy a whole team, but if the Impure Shot stacks were simply physical damage that could not crit, or if they counted as physical damage during the ultimate, it would allow armor pen to help her out far more, and give her Black Cleaver synergy once again.
The changes they did for her last year were excellent overall in my opinion, but the item rework shortly after her changes were not positive for the changes itself.