I think taric is a good example of going from "too reliable" to "completely useless against anyone with a brain". The only people you will reliably hit with that thing now are bad players or champs who are as immobile as you are so only the absolute bottom of the food chain.
And this is really heartbreaking because I quite like the way tarics stun works. The fact that it is telegraphed and can potentially cover a large area while taking your teammates positions into account can make for some very satisfying plays, but in modern league you are almost never in a position where this can happen consistently. Now you are either matched against immobile champs where your stun is super powerful to the point where it feels unfair or you face mobile champs who will never get hit by it which also feels unfair. ( and don´t forget that the ratio, least to me, feels like 15% immobile vs 85% mobile )
So it is like you said. For immobile champs basically, nothing has changed but mobile ones have it a lot easier now. And like always, it´s all to promote more movement to create the illusion of complexity. And one of the main reasons there are so many supporters to this trend is that most people just want to be one of these hypermobile jackasses and don´t really care how this impacts the design of other champs.
eventually, riot will have updated every champ to have insane mobility. It will play like shit but oh boy will it look flashy.