You are right, they do have different unit stats. Things like str, constitution and other similar stats differ between the basic units. Not only should this information be available when you select custom factions, this information should also be available when you select sovereign+faction before starting a game.
I think you should move this topic to the Fallen Echantress Beta forum, by the way. Go to Edit, then you can view the subcategories in the dropdown menu near the title.