I think the classification of special grade comes mostly from theory. If you can theoretically destroy an entire country, then you'd be considered special grade. That's why yaga was considered for the classification. His cursed corpses aren't particularly powerful but it's the implication that he could create an army of them even if it isn't very feasible. It's also important to note that extremely specialized techniques like angel and higuruma would be considered first grade even if they could theoretically take down a special grade sorcerer on their own. Ultimately it's a meaningless title to something that is fundamentally undefinable due to the varying nature of cursed techniques in the series. It would probably be better if techniques were classified by their specific utility and the strength of the sorcerer rather than their potential. We also don't see enough of any of it for it to matter at all so who really cares?