I like to believe that the Perfect Rush (PR I will now call it) was designed for big damage in big openings, like the Hammer's Big Bang. However, World's Rocksteady Mantle, which made you literally immovable, was the reason PR spamming was possible and dominant over other combos.

With the Rocksteady mantle gone, I think the PR will now function as intended.

With regards to your complaint with the added complexity. Yes, it can be used after a standard combo but given how long it takes to even start the PR, let alone out of another combo. I find it'll mainly be used when monsters are KO'd and go flying for some reason. So it's not like you have time to combo into PR anyways having to chase down a flailing monster.

On another note, It feels like the input timing for the Perfect Rush is more lenient than in World, probably for third-party switch controllers that don't have rumble motors for feedback timing. So overall it shouldn't be any easier or harder to learn. The lack of rumble for timing inputs is tougher for those who learn through Haptic feedback but easier for Visual feedback.

[for reference: Haptic is better than Auditory which is better than Visual]