There is an item that is only viable on one champion.
After the reworks of Yorick and Urgot and changes to Jayce and even Corki (going way back here), nobody seems to buy it anymore.
I am talking of course about Manamune.
There are enough AD-casters.
Why would this item not be good on Miss Fortune, Kog'Maw, even Jinx (cause she can stack it with her rockets so easily)
Or Spell-heavy Melee bruisers? Darius, Fiora also have an easy time stacking it up.
Well, I have done the math and it just doesn't work. Almost nobody gets enough AD and on-hit damage to make the purchase worth it.
Miss Fortune actually does considerable damage with her Q on-hit, so Muramana damage does mathematically pay off when completed, but still it is probably better just to stack lethality and snowball your lane.
This item should be reworked, so that it is a viable scaling option for ADCs to go for, if you can afford to delay your power spike.
But the way the item is now, you can stack the tear all you want, in the end the damage is mediocre at best and will only delay purchasing lifesteal and armor pen, which just give way more bang for your buck.