The weak point is locked to a cardinal direction, so it's not "on their back" like Shaco's, it's more "up, down, left, right". I agree it can be a pretty powerful zoning tool, but it does take a bit of effort (unless you use your Q) to get around your opponent and take advantage of a weak point, especially if your opponent is positioning well (the slows and speed boosts certainly help though). There are ways around it, like if the weak point is facing you and you proc it, then Q behind them to proc the next one, but it puts you in a pretty aggressive/vulnerable position in lane.
Regarding tanks, tanks also generally have CC to inhibit Fiora's ability to get around them and target weak points. I feel like more of Fiora's match ups now come down to player skill. I do feel, just from my limited experience, that the numbers are a tad overtuned, but I was also playing against bots (just to test her out), I imagine against an intelligent opponent it will be much more difficult to hit weak points, especially multiple times in a row (though it might take some experience playing against Fiora to be practiced in minding your weak points; it's a pretty unique mechanic and really changes up how lane dynamics).
Generally she feels like she'll be higher risk, higher reward; champions she used to be able to bully she'll probably still be able to bully, vs other bullies/tanks it'll come down to skill a lot more I think (she's squishier because she lacks her old HP regen passive, but if played well she can outplay). She's not like Fizz or someone who just gets %hp damage for free, she has to work a bit more for hers, and I feel like it'll make for more interesting lane interaction and fights, though of course this is with pretty limited experience on her, so take what I say with a grain of salt. I suspect, though, there will still be some complaints about her simply from people who refuse to change how they play vs her passive, but that's Boards/Reddit, lol
Also, how does Fiddle counter Fiora? I've never heard of that match up