MF is incredibly strong, but also has real weaknesses:
Almost all of her teamfight damage is in her ultimate - which can be quickly negated with cc. I play J4 and ahri and in both cases when MF ults and everyone is trying to walk out while dying, i dive on her and hit cc and blow her up; it works more often than not (and if she's positioned far enough away to not be dive-able quickly enough, you're far enough away to get out of her ult).
Like jinx, her 2v2 laning is strong because she can trade well, BUT she's actually a weak laner because she's EXTREMELY susceptible to ganks. She has nothing beyond flash to help her escape (i don't really count MS boosts unless they're strong and reliable). MF lanes can be shut down hard by the jungler and mid laner.
She's probably a tad OP (and it's no secret, she's played as adc in the NA LCS with lots of success) but really not unreasonable. Varus is really causing a lot more problems it seems. I think she does well in solo queue because no one ever seems to be ready for her ult and the panic reaction to run is almost always taken over the often better action of diving on the now unable to move adc (free 100-0 combo on their carry, why isn't this More obvious?)