From what I have observed owning and servicing a lot of both types, the 'shorter' life of the outboard depends on if the cartridges are properly greased from the get go. Sometimes a manufacturer, particularly an aftermarket can spec a generic bearing and not what grease is installed in said unit. That is clearly not as much the case with the older sealed cartridge bb assembly was not as dependent on pre-made cartridges therefore less variability. Every once in a while I have opened an external bb cartridge(and other places) and found a minimal amount of lubrication installed. As a response I have taken the policy to check when they are openable(of course) and supplement any periodic lacks.