Champions who rush mallet are too tanky to deal with, but you can't escape them once they start using basic attacks on you.
So we solve that problem, by giving mallet even more HP?
I frequently build mallet myself, and I can't force myself to see this as a nerf. 10 AD isn't really important in mallet builds, if 10 basic attacks didn't kill my enemy I got all the time in the world to land 11th one. 50 HP to make me even harder to burst down is more than welcome tho.
TequilaZombie2 votes
Is meant to be a tank/bruiser/jugger item. They are making the item more appealing to those classes, while nerfing it to it's abusers (mostly ADCs and Yasuo).
Zero Skill Tank1 votesThe thing is, replacing 10 ad with 50 HP in no way makes it less appealing for "abusers". If anything, it makes it more appealing for them, as it makes them even harder to focus down with additional HP. 10 AD isn't that important if you can order as many basic attacks as you wish - that's the whole point of mallet builds.
TequilaZombie3 votesIt becomes a bigger of a trade off. They are giving up on items like BT, IE, PD, and LW line for the slows. Before the reduction, it was a trade off of, at most, 35 AD (not to mention the multiplicative stats as AS and Crit chance/dmg which can make this 35 AD difference increase to great lenghts). Now the difference can reach 45 AD, almost a 
worth of AD.
Considering how reliant ADCs are on AD scaling stats such as Life Steal, Attack Speed, Critical Chance and Critical Damage, the 10 AD lost becomes even bigger, as it directly affects the usefulness of those stats.
Now put this item on a Darius. He's losing 10 dmg on his autos, some on his Q and Ult and that's it. Not that much of a change for him or other Juggers who already depended on FM.
TequilaZombie2 votesWith a full crit chance/dmg and full AS build, those 10 AD mean a 62,5 reduction to a character's DPS.
As Juggers usually stay away from AS, they barely reach the 1,00 AS mark. Not only that, but they don't usually build crit, meaning that those 10 points in AD reduce their DPS by less than 10 dmg, while giving them the much needed tankiness.
Zero Skill Tank1 votesThe most common abusers tho don't build too much AS or crit - frozen force Yasuo only gets some bonus AS, Gnar gets neither AS nor crit, Kog and Teemo build AS, but usually not crit - or 30% of it from Hurricane.
This is exactly what concerns me - this nerf seems to be aimed at crit marksman, but mallet is already pretty bad of an item on them, while most of current users barely feel it as a nerf.
Juggs could benefit from this change, but mallet is almost never build on them, as phage items are much more preferred to gain MS advantage.
Erockandroll1 votesThe most common abusers tho don't build too much AS or crit - frozen force Yasuo only gets some bonus AS, Gnar gets neither AS nor crit, Kog and Teemo build AS, but usually not crit - or 30% of it from Hurricane.
This is exactly what concerns me - this nerf seems to be aimed at crit marksman, but mallet is already pretty bad of an item on them, while most of current users barely feel it as a nerf.
Juggs could benefit from this change, but mallet is almost never build on them, as phage items are much more preferred to gain MS advantage.
No Idea when Kog and Teemo made use of such items. Gnar is exactly the kind of Tanky character that Mallet was built for. But this is still a nerf for Yasuo. More so now since he's being adjusted to be more crit and AS reliant.
And for Juggernauts. There's no reason you couldn't get both. And I usually enjoy getting a Frozen mallet if I needed to slow down my pray, if I'm playing a AS juggernaut such as Olaf.
TequilaZombie1 votesOn Kog and Teemo who want that sweet AS to proc their On-hits more often it's a potential reduction of 25 dmg on their DPS (I don't wanna go all mathy on Kog's W, but it should be a smaller reduction for when it activates).
Gnar is made to use Mallet and Mallet is meant for champions like him: champions that want to kite and avoid be kited (previously, this was Urgot's territory, like, WAAAAAAAAY back).
Frozen Force Yasuo is an abomination that should be regarded as AP Yi when it comes to item balance. In other words, it's so ridiculous that it doesn't even deserves a mention.