Okay so as with most sci-fi, It only really works within its own reality, Because if FTL coms or FTL travel time was shorter you could implement different options. But ultimately having a handful of barely overseen Mega-corporations (run by the high lords of Terra in theory) with theocratic leanings forcing a heavily decentralized confederation of planets into line is to ensure a united Humanity at all costs. At the end of the day its comes down to how much you would give up, In a situation that bad.
Yes, I think the Imperium of Warhammer 40,000 is a valid structure for governing humans, because the universe is just going to fall apart if someone is not in charge. The Imperium is a powerful organization, and capable of managing all the life forms in the universe and representing our galaxy's interests to the greater universe.
Where do I begin? Humans have been trying to find a form of government that works, but history bears out the truth that man was not created with the ability to govern himself. The only valid form of government is theocracy. What is a theocracy? It is God-rule, in other words ruler ship administered by man's creator. Humans have failed miserably at ruling themselves. We are now on the brink of destroying the Earth and no form of human rule has been able to bring peace and security to manking. Only God's Kingdom can accomplish that.
In the future, humans will design newer and more effective systems of government. Right now, one could say we live in a technocracy as the haves and have nots in society can either use or reject technology. The Imperium in Warhammer 40,000 isn't perfect, but for future humans it works. Hopefully, such a governmental system won't come about far into the future when humanity spreads to other worlds.
The Imperium in Warhammer 40k was modeled after the European church directed and monarch led time known as "the dark ages." As the name suggests, this was a time when science and art were suppressed. The evolution of society was brought to a halt, and only ancient practices and beliefs were accepted as viable ways to live. The Space Marines even wore ancient armor that they not only could not reproduce, but could not repair. Because there was no expansion of thought allowed, the space marines were doomed to eventually die out as they ran out of armor that allowed them to be such a formidable force.
This structure of governing is not reality, and is only a fantasy thought put into a video game. Although it could work, since there is hundreds of types of governments at use in today's world, I thing the government that the United States uses today is the fairest form around