Phase Rush has fundamentally failed on every level of its intended design

Riot's intended purpose for implenting the 3-hit proc from 30% max HP damage: "We wanted this keystone to be more accessible to bruisers, especially early game, and 3-hit proc was more consistent than 30% max HP damage. The 30% max HP damage made this rune rare to take on the old system."

The actual reality: SRS was already accessible to a number of melee champions and bruisers in both the laning phase and late game. The 3-hit proc has made this keystone completely undesirable to a number of melee champions that used to take it. SRS was an insanely popular keystone.

Riot's intended design for undertuning the numbers on Phase Rush: "We think this keystone has a lot of late game strength, and since we made it so much easier to proc earlier(not really) we're going to make a late game scaling rune."

The actual reality: Phase Rush is SLIGHTLY easier and more predictable to proc earlier in the game but it's late game is actually a LOT worse than SRS. You could easily proc SRS with 1-2 abilities by late game and it had a lower cooldown of 10 seconds. Now it doesn't matter how fed you are, or how many levels up you are, you are forced onto a 3-hit mechanic as any champion(this is especially annoying as a melee champion or a mage like Vladimir though).

Another part of Riot's intended design for keeping the numbers on Phase Rush low: "We wanted this to be viable for bruisers and melee champions without it being too strong on ranged champions and being forced to simply balance it around mages."

The actual reality: This keystone is virtually non-existent in the game except for 2-3 champions who regularly take it and only because they actually no better option(and they all happen to be ranged). They put the numbers so low to the point where virtually no melee are ever willing to take it above actual decent keystones like Kleptomany or Grasp Of The Undying.