For something like this, a simple answer is more than satisfactory. It was already given too. Assuming that it was the 4th Oro was summoning and that it would have been successful, Sarutobi would have had to fight 3 Hokages and a Sannin (One on the same level as a Kage). You can tack on anything you want to that but considering the 4th's abilities, The 3rd wouldn't have beaten them all. Again, this is all assuming it was the 4th that was being summoned.
The Shinigami does eat the souls of the user and the victim of the Shiki Fujin. You see it while the 3rd is pulling out Oro's soul.