It depends on the ability.
Fiora's Riposte is only reliable against CC with telegraphed animations such as Darius's E or Gnar's W.
CC with instantaneous effect, such as Udyr's Bear Stance AA or Riven's W are still decent against Fiora.
In order for Fiora to block and redirect these abilities, she will have to predict that her opponent is about to CC her. She cannot rely on the animation of these abilities to time her Riposte.
I find that hitting Fiora with CC point blank will almost always be effective. For example, rather than casting Malphite ult from half a screen away, walk up to her and use it point blank eccentrically in lane. If your CC has a obvious animation before you can cast it, such as Jax E or Udyr E, walk up to her with the animation, and bluff that you will CC her. If Bear Stance or Counter Strike is about to run out and she still hasn't cast Riposte yet, just back off.
Also, her W CD is around 20 seconds at rank 1. Her W is ultimately still a skill shot. Most people forget that they can just walk out of it and have their CC abilities back up before Fiora's W is off CD again.