Most people who play fiora are fiora mains, kind of like shaco. This is why it seems that Fioras always seem to play well in my opinion, in addition to her ability to snowball. Build zhonyas against her, it will stop her ult and really cut down her damage (Even building it as a defensive item on AD's isn't that bad).
She is also Feast or Famine, it is easy to talk about when a fiora has crushed a whole team with her ult but there are also many cases where fiora falls behind early and has a rough time coming back into the game. Given a few kills she will snowball, but starve her of that and she will not be much of a threat.