[glenneroo]

And if you pause sound in Unity using AudioListener.pause = true, which is supposed to make life easier, but ends up being useless if changing settings/clicking buttons has audio feedback, or changing the volume has audible feedback to tell you how loud, or you allow to change voice style, and on and on.

Repeat that for every system - all those edge cases for each system can waste a lot of time and energy.

[account42]

Like most "difficult" problems it's only difficult when you don't design for it from the start (including when you inherit design decisions by choosing a pre-made engine). If you design things properly you will have different time contexts for things inside the game simulation and UI - and that's useful even without pause as you'll generally want to control the timescales for those separately.