Quick question in regard to some champion designs.

So I noticed there's a handful of champions that don't have mana and no substitute for it either as well (Riven and Yasuo for example) while their cool downs don't seem particularly higher than average and they also seem to still be able to dish out a lot of damage and even have good mobility. Do these champions actually give up anything in exchange for not having to worry about mana management? If not, what is Riot's design philosophy or justification for giving these champs such an inherent edge over the competition?