I have been giving some thoughts to it, there are three kinds of Zoan categories introduced so far, right?
The ordinary Zoan, The Ancient Zoan and finally the Mythical Zoan.
Well, its been refereed as even rarer than Logias. By far we have only seen two users, Marco the Phoenix and Sengoku the Buddha. Both are extremely powerful figures with out a doubt, however there is some speculations going on about the Gorosei being all Mythical Zoan users, some have also predicted that Mythical Zoans were the key to win a supposed war that took between the founders of the World Government and the Ancient Kingdoms. Now, Oda himself declared that Logias are the most powerful category of devil fruits. (Mythical Zoans are rarer than Logias not necessarily stronger). He (Oda) also said that the Gura Gura no mi is comparable to Logias in strength (no comment made to Mythical Zoans). I agree that Mythical Zoans are not ordinary stuff and they are much stronger than ordinary Zoans but still they are not like the ultimate weapons, my guess they are as a class the second most powerful category, after Logias, however, there are some Paramecias too which can not be neglected, for instance the Gura Gura no mi, the Nikyu Nikyu no mi and the Doku Doku no mi. Also the user is a great factor, Buggy is a weaker figure however his Bara Bara no mi does show potential also Luffy's Gomu Gomu no mi is not that scary of an ability but Luffy sure made use of it. As for the Gorosei, they are the highest authority of the world, so if they are fighters (which they look like) they must of the highest caliber, possibly even more powerful than Admirals. Yes Mythical Zoans does seem like a good choice for their powers, but so does Logias or some Paramecias. My point is the every discussions about Mythical Zoans seem to overpower them. In my opinion, untill now the only fruit which could out match any other devil fruit and could be claimed to the JOKER OF THE DECK is Blackbeards Yami Yami no mi. So the Mythical Zoans were/are the key to World Governments strength does not seem logical to me.