Well, Sunni has the strength of being the biggest religion of the game, and a hard one to convert. Just from a conversion perspective, I'd rather be a Sunni conquering Europe than an European conquering the Muslim world because of that nasty -2% conversion strength modifier which makes it impossible to convert any Sunni land early on. And being Sunni meant you always had the Ottomans on your good side if you also rivalled Mamluks, every time I played in Africa I turned Sunni and not Coptic to ally the Ottomans which is the easy way to go, while Coptic is better but harder and more dangerous. Mechanics do not mean everything, it's also about political balance.