Masteries that add new mechanics or extraneous power to a champion are inherently game-breaking

This is especially evident in many keystones; allow me to explain.

Take Zac for example. Zac is essentially balanced around the fact that he has strong AoE damage and long-range engage. However, he uses health and is thus primarily a health-stacker in the vein of Tahm or Mundo, and so he is vulnerable to % damage. Once he goes in, he also has no real options to get out and limited defensive options outside of his situational healing, and so his engages are a usually surgical affair.

A mastery such as Courage of the Colossus adds an enormous amount of power to Zac. Once he goes in with his slew of AoE CC, he now has a massive shield to cover his lack of on-hand defensive tools. Teams that before could have sufficiently recovered and retaliated from his engage now have a much harder time, and Zac no longer pays as much for making a poor engage. This mastery shifts and or even totally destroys Zac's previous weaknesses and forces Riot to either nerf the mastery (which they have already done), or nerf Zac around said mastery, which is a betrayal of _champion _balance. Disclaimer: not saying Zac is either over or underpowered.

Other masteries such as Thhunderlord's, Fervor, and Grasp have proven or even continue to prove to be severe balancing issues in the past because they provide so much extraneous power in the form of extra damage and/or sustain. Champions are inherently balanced by how much damage they can dish out through their auto-attacks and spells, as well as their ways to protect themselves from other champions' damage. Adding additional damage, even in regular, non-keystone masteries, is dangerous because it can totally break a champion through their sheer damage output or defensive capabilities.

In short, keystone masteries that specifially grant new mechanics and/or vastly increased damage capabilities are absolutely capable of breaking a champion's balance state. This forces Riot to balance around these keystones instead of the champions themselves, a thing that very few, if any, MOBAs have to do. Half of the tanksassins wouldn't have worked half as well if it hadn't been for Grasp and Fervor; Brand would never have had his heyday had it not been for Deathfire Touch; alongside countless other abuse cases.

I would strongly advise Riot to reconsider how healthy keystones truly are for this game.