As both a player and GM, the various "Save Mastery" class features make the game feel less fun at higher levels, and I wish they worked differently.

Maybe I'm the odd one out, but I personally don't like that creatures with abilities or spells that target saves instead of AC become less and less of a threat the higher level the party gets, because of the "Success becomes Critical Success" upgrade that becoming a Master in save proficiency gives to PCs.

Most characters get this feature for two saves, Rogues for some reason get it on all three, and it's possible to kind-of-but-not really get three on other characters as well by using certain ancestry feats like Forlorn or Lab Rat. It really is quite widespread, considering how powerful it is in practice.

Critical Successes on saves are always the least interesting outcome, because literally nothing happens. This is true regardless of whether the critical success was rolled by a monster or by a PC. The game just becomes less interesting as a whole when player characters get to a point where they actually can't just roll a regular Success result any more against many abilities, in the same way that fighting a monster who critically succeeds on every save because of inflated stats or a big level gap becomes very frustrating and uninteresting a lot of the time.

Mind you, I would rather the saves system work like this than in a certain other RPG I won't name, where your character will likely fail several different types of saves against high level creatures even when they roll a 20... but it could still be better.