With the Juggernaut Update Riot starting trying to make older champions more current and fun to play than their original design allowed. Since then there has been an Marksmen, Assassin, Mage and now Tank Update. With every update some Champions succeed where others fail horribly.
If the Juggernaut's that were reworked are looked at everyone of them was pushed to fill a niche or a stereotype of some sort, even if that niche or stereo type had nothing to do with the original design. Darius was pushed to become a champion that wanted to be in the middle of a fight and loved extended trades. Both of these things were what the player base of the character, arguably, wanted to do more of. However Darius was also given more counterplay to hit kit, don't let him get 5 stacks and don't let him hit the edge of his Q. This meant that fighting Darius wasn't as boring and straight forward as before.
So Darius's update succeeded because players got to do more of what they loved, fighting and slam dunking people yet Darius was also given more counterplay. This allowed Darius to be a much healthier and balanced champ than before and a highly picked champion to this day.
Compare Darius to other Champ updates that failed, what did they do wrong?
Mordekaiser was shoved into a lane and position that none of his player base wanted. He was given abilities that interacted with teammates and the ability to control dead dragons. So he did get more ghosts to play with. Mordekaiser was given very little counterplay and none of his previous issues were really solved.
Malzahar become a minionmancer because his old passive occasionally game him minions. So he had his old passive taken and moved to a primary ability. A large part of his power budget was put into his Voidlings and his Voidlings became harder to control. The Voidlings changes on top of Malz's new passive were not things Malzahar players wanted. Not only that but both Malz and his opponents lost the ability to play around Malzahar because the AI and strength of the Voidlings now became key. Malzahar gained no counterplay and lost participation in fights.
Garen was given a new passive, true damage against Villians. He also gained the ability to do more damage while spinning. However he can't control who the villian is, nor was he given any way to reliably get to the villian. So unless Garen is actively fighting the villian he doesn't deal tons of damage, but if the enemy is the villian they stand little chance of beating Garen by themselves. Once again Garen gained no counterplay and lost input.
If you look forward at the upcoming tank rework Sejuani looks to be ready to repeat this problem. Her ult is losing much of its power and so is her empowered Auto. Instead she is getting the ability to have allies get an opponent ready to be stunned. Sejuani is both losing counter play and player agency, with one ability/passive getting most of her power. If she is released as is she will probably become another Malz, Morde or Garen, maybe even a Koggles.