Aside from his military prowess and suitability as a big picture campaign planner rather than mere battle leader he activly welcomes and takes part in administration. The friction between the civilian council and the military was one of the factors that led to the Heresy. Guilliman would be able to maintain much better relations with them, indeed he'd likely welcome them. His being the largest legion with its own pocket empire and independent supply base, he'd be well equipped to put down any legions who dissented.
He wouldn't just conquer an empire for humanity but build one.