actionman
Emperor Mongoose
Centralization in terms of concentration of power.
Decentralized, in the sense of delegation of responsibility and authority.
The more I look at the Imperial feudal system, the more effective it seems.
The honor nobles, who don't have fiefs, can be given fleet commands to keep the ranked nobles, who do have fiefs, under control.
The fleet commanders are supposed to obey the ranked noble subsector and sector dukes, but if those dukes command them to disobey the Emperor, the fleet commanders can ignore such orders with impunity. If necessary, the fleet commanders can arrest the dukes who give treasonous orders. To have a chance, all the subsector dukes would have to go to war in support of their sector duke, and they still wouldn't have the capital ships that the sector fleets have. Even then they still wouldn't have a chance, because the Emperor would order his sector fleets in nearby sectors to attack, and for good measure he could place the subsector fleets of nearby sectors directly under the command of the senior sector fleet admiral moving to put down the upstart sector. In one stroke the Emperor would assemble an overwhelming fleet and strip nearby sector dukes of the fleets they would need to support the upstart sector duke.
The reason fiefholding ranked nobles shouldn't have fleet command is because they need to be administrating their fiefs. Fleet command and fiefholding are two jobs that are so demanding that they can't be done by the same person, especially if there's a military operation that takes the admiral / fiefholder away from his fief duties for any length of time. A fiefholder who doesn't manage his fief would soon find his title for that fief revoked and given to someone else.
Honor nobles, who may be fleet commanders, can only receive fiefs from the Emperor. Therefore, it behooves them to be as loyal and effective as possible to prove themselves worthy of being rewarded with a landed title when they retire from military service.
All the incentives and military realities favor loyalty to the Emperor and don't favor rebellious vassals.
Enfeoffment and control of Imperial military power are fairly well-separated. He who controls worlds is not he who controls fleets. It's a pretty good check and balance.