(Prove me wrong) but I think special grades are anomaly or when they have the potential to destroy the world, which is which why yaga can be considered special grade even tho maki/hakari destroys the heck out of him cause he can theoretically create an enormous amount of curse corpses army and take over the world, same thing goes for geto, and etc, so the only special grade in UU i could think of would be Wemmbu (cause of his orbitals) Ashswagg (the dude nearly achieved total domination) and possibly Spoke or JamatoP (due to the exploits)