In my opinion, Fiora has a very powerful kit and I do agree she needs a nerf. Fiora with 300 ad has a passive that does 16% of the targets max hp per vital hit. She has her q that makes it very easy to hit the vitals especially with the ability prioritizing people affected by her r or passive. When you add armor pen and armor reduction to the mix her r and passive get even more broken. Her r that can be expected by its self if you hit all the vials to do 68.9% max hp (452 ad with a black cleaver and dusk for armor pen and reduction) can easily do 8192 damage to the training dummy. The dummy does have no armor but with Fiora's passive and ult damage type this doesn't change too much because her r and passive do % max hp true damage so both armor and hp won't change the outcome of the damage.
I would say the scaling of her passive should be reduced and changed from true damage to physical or do something else with her passive. If riot changed her passive her ult would also be affected because her ult just creates 4 passives that stick around and do a lot of damage if all 4 are hit.
Other than her passive/ult the only real thing that is unreasonable with her kit is her Ripost (w). Its ability to block all damage that isn't a tower. She can block damage from any direction and then use any cc to stun a person hit. I don't see why she is able to use her sword to block stuff like a Mord r, Rumble Fire, or a few other bits of damage in the league that a sword can block. From her lore, it can make minor sense because of Demacia being the anti-magic faction but still blocking fire from a flame-throwing that isn't magic shouldn't be blocked by a simple rapier.
These are my opinions and my view on the current Fiora. I understand the difficulty in managing balance and expectations in game development. What I have put here is my opinion on how I see Fiora and her current kit. I don't expect anything I put here to be used to change anything but I prefer to speak my part to start something.