Skarner wasn't fine before the rework. He has been the most reworked champion in the game and this is his second or third rework because something always comes up that makes him too OP and then RIOT has to gut him.
Darius did get changed this way. The problem is that he is just slightly on the strong side now, so he can stomp people who have no idea what they are doing anymore.
Morde got changed because he needed it. Getting moved to bot is good for him because he needs more safety with a support. He needs to be toned down, but NO ONE would play him if he was released being incredibly sub par.
Garen's changes were fine, he is basically the same, just has more killing power on specific targets.
Regardless of how strong/weak they were, there is no way for RIOT to be able to release them perfectly besides being lucky. As I have said before, it is better if they are on the stronger side because then it will make people play them because they are OP, letting RIOT get data on what is easily being abused on those champions kits so they can nerf them accordingly if it is needed. Also my understanding of the Juggernauts is that their strength comes from you running at them and them being able to beat you back. If you are playing into their strength and not trying to kite/poke them down, you deserve to lose as you are playing into their strengths.