Just a note:
in a game where the balance is as complicated as this one, everything is a factor. Not just national units, national spells, skills, national gem income, etc. But even something like the heroes they get. WorthyHeroes (which I totally love) can still arguably affect the vanilla balance altho probably not much. About the only "no effect" mod would be something like Streamers and Banners (altho there is some debate about the affect on players and enemies)
It also adds another layer of learning for new players. Along with email in and email out you might have install issues.