Or you can counter hr vitals with shields or some form of health gain.
If you take a look at her stats on Champion.gg her worst counters either have range, shields. or rapid health regeneration.
Fiora's Q works great if she can close the gap with one cast, but if the enemy is out of range, she will not be able to retaliate without taking more poke damage.
As well, shields and health regeneration hard counters Fiora's vitals. They calculate off of maximum health, so shields and any health gained during combat is not included. This is why champions such as Sion, Mundo, or Malphite has such absurd win rates against Fiora while she counters other tanks such as Cho'gath or Maokai.