At what point does "mastering the game and its mechanics in its entirety" stop and start becoming a waste of people's time? Because I'm talking about the unfair and time-wasting achievements. You can always keep making harder achievements and justify it with "I guess you didn't master the game", but the reality is that these achievements just taunt everyone who doesn't want to dedicate thousands of hours into the game. Achievements should be reasonable.

You could make an achievement for beating the game on the hardest difficulty. Then beating it on the hardest difficulty with the true ending. Then beating it on the hardest difficulty without dying. Then without getting hit. Then beating the true final boss at level 1 with a butter knife without taking a single hit within a 10 minute timer and you need frame-perfect jumps to avoid supposedly undodgeable attacks.

The first is fine, the last is a complete waste of time and shouldn't exist. This isn't some weird undocumented phenomenon, adding ultra-hard achievements will often lead to people burning out trying to achieve it and people complain about them all the time. If you want to do a challenge run in your own time, feel free, but I guess I'll die on this hill that they shouldn't be an official achievement.