Are Keystones a faliure?
As of now I think they are and I would personally like to see them phased out
they never have been balanced and I doubt they ever will... with such a large champion pool things that buff certain effects and play styles so considerably are going to always create a glaring power discrepancy throughout the full pool and will more than likely always find ways to give unwanted power to champions that shouldn't use them(or excessive power to champions that should)
Fervor on tank assassins Original bloodlust on Tryn and Yasuo CotC on Vi on Poppy League of Thunderlords Grasp on GP and Gnar SotA in the Jungle(specifically Rek & Grag) etc.
also they generate balancing nightmares
such as this situation Champion X abuses Keystone Y
ok nerf Champ X to balance them...
patch or 2 later nerf Keystone Y becasue it is still too strong on other champions of that class
now Champion X is totally dumpstered