Board Thread:Naruto Discussions/@comment-30323490-20180418150329/@comment-30323490-20180418200715

EMS works differently. It doesn't increase you chakra pool by any means.EMS Sasuke had the same amount of chakra as MS Sasuke. He could conjure the Susanoo because of his eyes and not so much because of his chakra. Itachi had close to no chakra and yet still conjured his Susanoo which was Hydra-Orochimaru size.

And it obviously wasn't impossible to summon the 9 bijuu at once, but the amount of force and chakra required must have been ridiculous. The 9 Bijuu together + the Gedo Mazo make up the ten-tails. The Gedo Mazo is absent, but in exchange you get another 4 shinobi that you need to summon. So you pretty much need to summon a ten-tails league lot. Not to mention that What Hagormo did was no regular summoning. There was no blood contract involved. This probably also made it harder for him, hence why again, he needed aid from the other Kage.