Passer rating, to me, has always served as a general “efficiency” metric. There’s an argument that completions are weighted too heavily, but as we know, most WR average over 10 yards per catch and running backs anywhere from 6 to 8, so completions usually equal extending drives.
As with any efficiency metric in sports, there’s a way to game the system to inflate the metric (namely, checking down and taking safe completions). That doesn’t make the metric invalid, it makes it imperfect.