I do not play her (or even the marksmen role) but I do understand how she works (because I play support) and I have seen my fair share of good MFs, poor MFs, Great and the bad.
I have seen, what I categorized, two different builds with her. AD MF and Attack Speed MF.
AD Miss Fortune is exactly that, getting 5 items all for the sake of AD and becoming a nuisance in power to Everyone. The only attack speed she gets is Berserker's Greaves and using her w Strut. With items like The Black Cleaver and Duskblade of Draktharr (Well... before pre-season 7 because I feel like the new passive won't work well with her but what do I know? We'll see) being part of the build. Good a minion clear she is not (I have literally seen a MF ULT a minion wave just to clear it [I guess I should specify that the minion wave was a wave left alone with about 20 coming to your turret, dealing 75-100% turret health damage]) but murdering everyone is her specialty.
Then there is Attack Speed Misfortune. Which consist of getting attack speed items (No duh) and critical strike items. This Miss Fortune is the ADC we all expect. Eas(ier) to kill and better at minion wave clearing. Assming the crits don't get you, being killed by this build is harder.
Anyway, the point is, the insane damage partly depends on the build and also on the fact she is powerful.
Knowing Riot, they would argue she is balanced because she has no means of escaping once hit. Because movement speed is gone. Actually, expect a buff with Miss Fortune Support ruining her gameplay (or did it recover since then?).