Now what in the manga has ever said that sasunoo is stronger then madaras current defense. If sasunoo was stronger and he had the ability to use it, dont you think he would do that instead? In fact if regular ems techs are better then juubi mode techs why go through all the trouble, it seems smarter to stay as a edo and use sasunoo, kill everyone tyat stands in your way and then start the moons eye plan. Sasunoo is too big and too slow its been proven to not be unstoppable, the raikage destroyed one sasunoo (even though it wasnt the samurai version), thoughs black balls protected juubi obito from a tbb and more, more then likely 1 or 2 tbb would destroy samurai sasunoo