Fiora has always seemed really stupid to me. She's so strong for how low her skill ceiling is, or how few champ-specific mechanics she has. She's basically just really good stats with 3 steroids, 2 of which cost no resources and one of those being permanent, and a targeted AOE nuke that makes her invincible.
I think melee ADCs are just poorly designed in general, that's why they're the most hated champs in the game. If you look at all the champs that are universally hated with the exception of Teemo, they're all melee ADCs (Trynd, Yi, Fiora) or at least melee carry types ( eg.Nasus, Riven). I think the reason they're hated so much is because they have lower skill ceilings. They normally have really good stats plus some really brainless mechanic like a revive or invincibility takes very little skill to use.
I think people don't hate marksmen that much because they still take mechanics to use when they're fed, whereas fed melee carries can just facetank 3-4 people and get a quadra just be clicking R and using one of their easy mechanics.
Inb4bronzescrub