The complete role PM UncleSam posted is correct. The game was highly imbalanced (Samus was stronger than most games' wolves), but almost as annoying was the fact that the two main conflicts were almost completely independent. The Galactic Federation and its enemies weren't included in any Sphere's win condition, and the Spheres weren't in any non-sphere win condition. The only crossover between the two conflicts was that MB and Phantoon could recruit Spheres.
But I won, so who am I to complain?