Riot, what determines which champs are considered "healthy" and others aren't

For instance, lets take a look at and

yes, they are both AD oriented champs with assassin playstyles. They both tend to stack AD and Armor Pen, pick one target and destroy that target. They are also very strong in 1v1s and act as split pushers.

But has seen a long string of nerfs to his playstyle as an assassin and splitpusher, with Riot trying to turn him into a bruiser without giving him any bruiser tools. They say that his assassin playstyle has no counter play and that his splitpush is too safe. So now his burst is reduced and his splitpush is non-existent. Because Riot said that he was a "toxic" example of assassin's and their lack of counterplay and that there should be more involved with a splitpush than "push towers, then stealth and run away if they find you."

Yet has been called "healthy" by Riot. But he too stacks AD and armor pen and then ults his target and kills them in < 1 second. Either that or he splitpushes and takes towers and is pretty safe doing it. He can go over walls with his W; or even 2v1 by taking out 1 person with his ult within 3 seconds and then 1v1ing the other one. But, as I said, viewed as healthy by Riot. They nerfed his split-push with a minor AS nerf. Other than that, untouched.

I have nothing against or or really any champ, except for and an irrational hatred of . All champs have their strengths and weaknesses, and fotm periods, but this is my real question;

Why are some champs viewed as "healthy" by the balance team but other champs who fill the exact same niche are viewed as toxic examples of the same play style.

Other examples of this; Instant Stuns: vs Mobile Burst Mages vs DOT Mage vs Mobility on a utility ADC vs