That's the problem with centralized states, they are overrated as hell.Same thing occurred in China. In the North-South Dynasty period of China where the aristocracy still exist, Chinese armies were able to resist barbarians well and even periodically reconquered parts of Northern China. The Song and Ming Dynasty by contrast,where the aristocracy was replaced by the bureaucracy, no such thing occurred and both Dynasties saw little effective resistance despite their size and economy. In a centralized state,there was no obligation on the part of local elites to defend the state because they can just switch a master and still pay taxes. In a feudal state where obligations and duties are stressed between the ruler and the local elite, the local elite are obliged to defend the state either out of a sense of honour or to protect such rights which may not be honored by a new conqueror.
In the case of Chinese states, they tended to be a bit more successful in conquests ultra-China. There was also an undeniable notion within China regarding the divine mandate and how it related to the rejection of unjust and tyrannical rule of the bureaucracy. Byzantium on the otherhand, had no such conception. Rather, they had all of these competing factions who brought little positive benefits, yet were able to implement every negative policy and condition that their faction represented.
Bureaucracy: The only major positive I can see, was that the Byzantine policy regarding the movement of peoples to and fro in Anatolia and Europe, tended to be effective in quelling revolt and populating various areas of interest. Other than this, we have only negatives. Bureaucratic states or statist elements can have the effect of preferring either; standing armies of less than adequately trained warrior who were operatives of the state or mercenary armies who were bought and sold as pleased, but posed little threat to the ruling class. In the case of Byzantium, we find that this is consistently a common theme when not under extremely skilled militarist emperors. Some scholars, such as John Julius Norwich, have described the bureaucracy as having an unbridled hatred and fear of all things military, especially the nobles of Anatolia. They were paranoid of the Iconoclast militarist schemes and feared too that fetish of military would allow constant change of the guard. The bureaucracy also constantly flowed toward centrality of imperial custom and attempted to defang any local rule or powerbase and wished to displace said peoples and move them across the empire in haphazard ways. Their issue too, was focusing on how to centralize power fully in what they owned currently and attempting to apply notions of state that worked in Constantinople to rural areas; the ivory tower of Constantinople as some have termed it.
Military: In contrast to the Bureaucracy, who wished to rule from Constantinople and effectively manage the populace, we have a military that viewed the empire from the frontlines of war. This is most exhibited in the Iconoclast heresy and the militarist emperors who viewed the empire entirely different. They wished to expand the empire and recreate it into something that could run in a changing world. They however, in their methods became infamous. Completely alienating the population of Greece, by destroying monasteries, relics, raising taxes for war constantly and challenging the Papacy, was a losing proposition. These emperors found their beginning in images such as Heraclius, supposed heroic figures to whom they regarded as figures of divine kingship. Who wished at all costs to both increase imperial power, military strength and to compromise with the southern and eastern elements of the empire, even at the cost of its western base (Anatolia-Greece-Sicily-etc). Meanwhile, as the militarist emperors attempted to expand the empire, the bureaucracy behind the lines attempted to apply Constantinople and their policies and hampered existing systems, while the militarist emperors did the same, disallowing positives of bureaucratic empires.
In later times we have the aristocratic land lords of Anatolia who become prominent and are part of the general discontent. Unwilling to submit to either force despite their creation at the behest of one of these sorts of agencies.
Frankly, there was a centralism, but of the worst kind, where factions aspire to total centralism but achieve complete hierarchical disunity except when a stellar emperor is on the throne.