I just tend to completely forget the select-clipboard even exists - right until a HN submission reminds me, or I drop some garbage in the middle of the text I'm writing.

Text-wise the only thing I can think of where clipboards don't Just Work is indeed terminals, but I hardly consider that an issue in practice. Either it's a trivial session and I'll happily right-click to paste, or I'm in tmux and using lead key prefixes for shortcuts already.

Cross-app rich media copy/paste does have a habit of being a bit buggy from time to time, though...