I had more bugs pop up from people calling front() on an empty vector than from direct out of range bounds. I guess people are more cautious when they are manipulating indices than when calling a method that takes 0 arguments?

Debug asserts are nice, but not sufficient when the code path causing the empty vec is only encountered by a production user from a different country at 2am. Enabling the asserts in release mode will at least prevent undefined behaviour and malevolent users from harming the system, but the best interface is the one that returns an optional, so as to make the "vector was empty" case explicit. That way it has a higher chance to be considered by the authors of the code or their reviewers.