Miss Fortune vs The World/Corki/Kog'Maw/Ezreal.
Let's just start off to getting to the heart of this particular discussion. Miss Fortune's awkward kid. The way it works in scaling in damage type and why it just isn't very good. Where as she might be compared to champions who also share magic damage strong kits who are good.
Why is it that Miss Fortune's kit just doesn't work? It's due to it lacking 2 things that the others have. Burst damage and poke damage.
Take a look at any of the other 3 champions (Corki, Kog'Maw, Ezreal). They build AD and their magic damage components can be used to poke down their opponents. Or if they get fed can still be used as burst. And if they build AP they trade that auto attack damage for incredible burst damage.
But why doesn't Miss Fortune have this?
Her only real poke is in her Q. Which is such an awkward ability and low in range. Corki and Kog'Maw can poke you with their ults at incredibly safe differences. Ezreal's long range Q/W leave him poking for days. And Corki/Ezreal have those escapes to boot. Miss Fortune's Q range is just 100 above her already low auto attack range. Leaving her unable to safely poke.
What about Make It Rain for AP Miss Fortune? It would work, except for three things. It has a long cooldown. It does damage over time. And it requires your opponent to stand within a specific area. This plays back into burst damage.
Miss Fortune's only real burst is her R or her E/R combo in Mage Fortune. But then lays the problem that once again, your enemy has to be standing in this one specific area for it to work. Unless your team is made out of lockdown your "burst" can be canceled simply by the presence of mobility creep.
AP Ezreal and Kog'Maw have made some fame for their burst and even Corki can be nasty to deal with once he's got his build going. They instantly burst their enemies down, not pray their enemy stands still for 3 seconds. It overall leads to a weak and gimmicky build for both AD and AP Miss Fortune.
And lastly, I want to talk about her W. This "building on magic damage." Let's just get one thing straight. This is useless on AP MF. It has no base amount anymore. Simply 6% of your AD per stack. While you do have magic pen it wont actually do any damage to matter. And while it seems nice for mixed damage on AD MF. Once your enemy gets magic resist this damage becomes completely ignorable.
But what about Kog'Maw? His W is fairly similar, yet incredibly better. First, it scales off AP. So regardless of build path this is going to be good. Secondly, it does health percentage damage. So even if your opponent does get magic resist they're still going to be feeling some effect. And lastly, it increases his range. Kog'Maw increases his attack range allows him to get off consistent damage at a safer distance. Where as Miss Fortune has to get into engage range of her enemies just to land a single auto.
Miss Fortune is simply outclassed. Her mediocre kit and illogical scalings leave her as a sham of a champion. She doesn't have the scaling of an AD carry and she doesn't have the poke or burst damage of an AP carry. She's overall weak.
I can't wait to see what changes they have in store for her. No more gimmicks Riot. It's time to face the wrath of the bounty hunter. Not the tickle of the Inconsistent Damages.