But Imperium of Man from Warhammer 40k worked
Its interesting you sayintg that because the Imperium come about after the Dark Age of Technology that is left purposely vague and, of course, the Age of Strife that is were human civilization collapsed.
The Imperium is complex on itself since are we talking about what time since it changed, back when Big-E was walking around there was no Imperial Creed officially and no Ecclesiarchy, the Inquisition was also formed after the Horus Heresy so it wouldnt happened again so we have the 30k of the Great Crusade Imperium and then the 41k Imperium that is diferent.
But some things are the same, the Imperium for example doesnt really care how a world governs itself and leaves it to its own devices as long they pay the Imperial Thine and dont do something stupid like worship Chaos for the obvious reason, there is a remarkable amount of leeway even if "legally" that isnt allowed, such as trading that happens because Terra is far away and the Administratum will only look more deeply if something is missing ...
This is not to say the Imperium of Man is good, even the Emperor only build it as a framework for his great work, the Webway Project that was meant to elevate Mankind to his level ... of course Magnus then screwed everything and we had the Horus Heresey ... the Imperium does work, to keep Humanity from being destroyed that is because after Horus "disagreement" its really just holding up everything from collapsing and if it does ... you rather live in a Hive or a Daemon World? As bad a Hive can be, there is far worst ...