Supports are what makes this game. Obviously ibaraki will be cheaper to invest in as a 4 star zerk. However raikou has the added benefit of being a buster crit Aoe berserker with a star drop rate on her NP. Raikou normal buster cards plus her mystic slayer and mana burst empowered crits melt mobs and bosses alike. Her NP is not the majority of her damage but her buster brave chain crits doing the work.
Ibaraki has no evade, less damage when properly buffed and only has her NP debuff going for her. Not saying Banana Oni is bad but Mom does too many things better than her. 5 star have their rarity and.cost for a reason.