Absinthe, half the reasons why casters are overpowered don't really apply to Kingmaker, or to videogame adaptations of D&D in general. CRPGs have a much narrower scope than actual PnP campaigns, and spellcasters can't use their spells in creative ways.
More fun to use? Probably, since casters generally have more "moves" and "actions" at their disposal. Stronger? No, because, as
Desiderius loves to point out, Kingmaker is a videogame based on party compositions and on interactions between characters, so the value that each class brings to the table is higher than its inherent abilities. In this game (especially at higher difficulties), you have to hit things to win, and a group with a Fighter and a Wizard is better at hitting things than a group with two Wizards, despite the inherent value of each class.