From what I read in your initial post about Romans vs barbarians. Perhaps romans cost more to build, and take longer, but you get 5 at a time, instead of 1 when they come out.
Then perhaps barbarians are created faster, but you have less control for instance, instead of being able to target a building or unit with a barbarian, you can just target an area, and then the barbarian will attack what ever they want in that location. The idea being that the romans won with banding, lines and organization. Where as the barbarians simply charged, attacking anything.