The web is meant to be a universal medium, agnostic in terms of device or UA. Of course, testing in several browsers is needed to iron out bugs. But when authors have to write for one browser, then port to a few other selected ones, something is wrong.
All in all, there is no simple right way for authors to use prefixes as they currently are.
CSS vendor prefixes were designed to help web developers by giving them a way to target CSS to specific browsers and use proposed standards before they were finalized. The idea was to move the web forward without rushing the CSS standards process. Unfortunately, it hasn’t always worked out that way.