Only revisionist history tell tales of flourishing kingdoms under a just king. In reality, the reason feodality worked for so long was the anarchy and power struggle, the cavalcades (basically raids) and a honour based justice (basically don't kill fellow nobility during war, and avoid killing militantes during cavalcades and you'll be good). The anarchical nature of the system made it particularly susceptible to organised raids, but also extremely 'agile' in it's political responses. Once power was consolidated however, the clergy and the royalty pushed their law and hierarchical order onto the mostly aristocratic feodality, it broke and you get the crusade against Alby, the war between Plantagenet and capetiens, and probably a lot of other misery inflicted to the general population. Then once the hierarchical order is set, you need an administration, which will become inefficient by nature.