Fervor skews the power curve of ADCs in an unhealthy way
late game ADCs in particular get far too much mileage of this mastery.
Previously, the way things generally worked was that most ADCs were lacking in early game damage,
being the exception as the lane bullies that would fall off later on.
Ashe, Kog'Maw, Jinx, Vayne, Twitch, etc, were pretty low on their early damage and relied more on their supports either having the damage to make up for their weak early or being able to ensure they could survive lane. Fervor has changed this entirely. Now, Jinx is not only able to trade in lane, she's a more than competent bully herself. None of these champions need to rush Infinity Edge and be required to build at least one more item before they can be useful; only an attack speed item
is necessary for attack-speed carries (Ghostblade, in Twitch's case), and they will benefit from it more than they would an IE into Runaan's. Now, 
Even more obnoxious is that it gives_ every _ADC a better late game than they've ever had. In addition to their high damage crits, they've got an extra 112 on-hit piled on, making
even more effective at shredding entire teams with rightclicks. Early game bullies and mid-game casters no longer fall off, even if they never get ahead; late game carries
power-spike hard from just one item, and only get stronger with more.
TL;DR this huge boost in ADC DPS is bad for the game and skewers their power curve such that their supposed 'weak points' are never actually realized.

builds Arm Pen but overwhelmingly takes Deathfire Touch.
s art going triforce into crit yet takes Fervor 84% percent of the time. And while Pen is the hot trend for
, he never takes anything but Deathfire.
, you have to have an MR item
.
. A damage item so you're relevant
. With just that itemization, you're still getting melted, despite all items having HP, and two of them having armor. With another armor item and tabi, it barely gets better.
superseded all other armor loss (provided it's greater than any others that might have been purchased), it would be fine.