Why assassins can't be balanced as easily as other characters, and why reworks happen.

An assassin's goal is to kill someone. Plain and simple, they don't do anything else too effectively. A Juggernaut deals a large amount of aoe damage to a team, or just stomps through teamfights if they play effecvtively. A Mage either brings a large amount of utility or magic damage. An Engage character either brings their engage to start a fight, or disengage a fight. An ADC brings a lot of single target sustained damage to targets that get in their range.

None of these character's goal is to kill someone, it's to set up a perfect teamfight. While assassins add to this, they do it in the most visceral and detrimental way, they turn a fight into a 4v5.

If an assassin can't do their job of killing a single target, and sometimes it doesn't even matter who it is if he has a really good team, then they become useless to a team. This makes the game a 5v4. That is their inherit risk, if the enemy team is skilled enough, or if their allied team isn't skilled enough, they can't do anything. There are fringe cases, such as when an assassin is fed, but you can't balance a champion around being fed because of the fact that no one can guarantee becoming fed. An assassin needs both levels AND items in order to do their damage, which makes them snowball stronger than other classes. A tank doesn't need the gold as much because their goal is to throw out their utility until they die, and protect their allies. A Juggernaut needs levels over gold because of how high their base damages usually are, that's how they're balanced towards late game.

The problem comes when an assassin actually tries to do their job. Rengar jumps onto your backline, and blows them up. Zed jumps onto your backline, and blows them up. Evelynn comes out of nowhere and blows you up! etc, etc.

But this job, blowing someone up, or multiple people if you're fed/ a good Katarina, can only be done with damage. Riot can't an assassin by giving them utility, like they can with a mage. The damage is what they need, over utility/cc. Nerfing the damage they already do can be catastrophic to the champion. There were two points in time where Rengar was worthless as a champion, his release and his big nerf. There was a point in time where Evelynn did no damage at ANY point in the game. In order to make this champion worth playing again, they can't give them cc and utility, because that doesn't fix their problems. They have to either put them where they were or slightly change their scaling/base damages.

Breaking Rengar's legs won't help, because there's no way to fix him in a quick meaningful way without simply moving him back to where he was to begin with. You have to change a character like Rengar's scalings to make him stay meaningful, while keeping his ability to be playable. This changes his item costs to be more expensive, he gets more from items than levels, which means he's more late game than anything, and he can still do his job at late game. Riot is on the correct path with Rengar, giving more counterplay to his ult and increasing his scalings while decreasing his base damages give him an effective nerf, and keep him playable.

TL;DR? Riot can't just nerf an assassin by changing their base damages. They have to change their scalings or make their items more expensive, which effects the other champions in the game, which is possibly the worst idea. The best point for a champion in Riot's eyes is when they are playable and fit into a meta. In the worst case scenarios, such as Yorick or Taric, where they either never fit into a meta, or are just bad in comparison to other champions, they have to rework the champion. Shen didn't work as well as he could've, so they made him better. Poppy didn't fit into anything because her kit was legitimately too strong, so they fixed her.

Rengar fits into the game, doesn't break anything, and is fixable.

Zed fits into the game, doesn't break anything, and is fixable.

If either of these champions were to break something such as abuse a new item, they can lower the item's damage, or they can lower their damage.

They had to nerf an entire STAT because of Vladimir, so that's why he's being reworked.

If only 40 champions worked in the game, that would be a bad game, currently it's around 90-115+ champions that work very well in the meta in the right hands. This works, this is fine, but it could be better. This was originally about the assassins, and how people don't want certain assassins to be playable, but I guess it kinda turned into why champions should be viable and why're they're reworked. lol.