So from the perspective of someone who plays her quite a bit -- she can be annoying if you let her be. The thing with Fiora is that she isn't exactly super strong in the laning phase. She does scale well but then unfortunately she also doesn't have the greatest team fight as she isn't tanky, doesn't have guaranteed hard CC, her AOE heal is not reliable, and while the dash has a low CD the range is super low that you aren't gonna jump on someone being peeled.
She works a lot like a Tryn -- once the laning phase is over she needs to split push over and over because realistically hardly anyone can outduel her although Tryn and and Jax tend to give me trouble with equal gold post 3 items. You counter her the same way you counter any split pushers.