Bringing his full might to bear, Shuma could lay waste to all of Asgaurd in a blink of its great eye. Shuma's ability to project it's limitless might far from it's home realm has limitations, however. It is limited by the mystic power of it's conduit or host. This is why Shuma-Gorath's possession of the Ancient One would have been so disastrous. The former Sorcerer Supreme would have been like an oil-pipeline of transcendent evil into our reality.
It bears noting though that a previous and less effective incursion into our realm was stopped by Crom (CROM!) during the Hyborean Age. If Odin could face Shuma-Gorath before he fully took root in our world, he might have a chance.