Critical CSS

I guess this just assumes that this is the first view of your page and no user has css resources cached?

Or maybe they are saying this would always be worth it?

I assume it'd be a trade off between a number of factors. How many returning vs new visitors? Is css served with proper cache-control headers, 103 early hints and in a cdn? How big is your critical css, and how much of your critical html does it push out of the initial congestion window?

Yes, for first view page and it is a trade off, best approach is writing all the styles and code yourself, not use libraries, etc.