The "traditional" method was to use a 4-square screwed to a piece of lumber. Over the years, I spent many a hot day crawling attics and wrestling insulation in order to instal that piece of 2x between the rafters. I never recall there ever being any issues with such installs.
Yet, it seems this was not good enough for some. Or, perhaps, others were not so conscientious, and used other methods. In any event, the code was changed (1999?) to require the use of rated boxed for fan support.
Metal boxes might be rated- but, as UL points out, the mud rings have not been evaluated. There lies the weak spot. That is, "fan boxes" typically use #10 screws to support the fan, while the device screws typically used with round-opening mud rings are but #8. As best I can tell, no one has ever performed the UL tests on the smaller screws.
I should point out that this is one area where the USA differs with Canada, as Canada requires additional support as well.
Your boss may be technically "outdated," but he sounds like a conscientious guy. After all, he COULD be using a rated 'pancake' box, leaving you to wonder about conductor fill, there being nowhere for the screws to go, etc.
