not as many people play her.
Well there's a reason for that. She's decent for solo carry, but... well shrugs
Her win rate right now looks to be 52%, which is slightly above average but relatively healthy. To be fair, a lot of games I've seen in posts with screenshots when a Fiora wins typically shows nobody building thornmail or the top laner not building frozen heart or thornmail. She can splitpush for days, but can't really run all that well...
Her pick rate isn't great at 4%, putting her as the 15th played top laner so it's not like she's being abused to hell.
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I don't know, I never really had issues with her. She has her counterplay (thornmail and frozen heart, general tankiness), she's squishy if she builds pure damage, has no utility or cc in teamfights and can easily be focused because she has to either stay on the frontlines (before/after ulting leaves her vunerable if you catch her before she runs) or has to run to the back (where she can't do anything). Her ult making her untargetable is annoying, but it's a huge part of her "power budget" (her passive isn't great, her Q can only be used offensively, her W has to be timed against autoattacks, and her E is great for pushing).
I find that her best counters are tanks and bruisers to counteract her glass cannon duelist fighting style. You win trades by baiting her W, hitting her with a spell instead of an auto, and only autoing her after it goes on cooldown. If you don't give her kills early, she's forced to roam or stay and cs, neither of which helps her team if you're on your mia ping and warding game. And if you're tanky and can hold your lane just fine, she won't be able to take your tower and further puts her team behind.
How does that help? A Fiora that doesn't snowball and deal a lot of damage is less likely to faceroll your team (just like Katarina and the rest).
Also, junglers should be camping top lane to set her behind. Seriously.
And consider picking up
against a Fiora. Don't die, build health and thornmail, roam with your ult, and teleport back into lane to get cs (and if Fiora takes ignite instead of teleport... well, that sets her team further behind if you deny kills).
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It's a bit long, but I hope that helps. I know people are frustrated with some champions
beyond just statistics, so I'm just trying to dish out some hopefully helpful advice.