According to Bentley and Ziegler's Traditions & Encounters, what were the dangers of the central lord-vassal relationship of feudalism?
The main danger of this form of political organization was that it was so decentralized. According to Bentley and Ziegler (p. 305 in the Brief Second Edition), “this decentralized political order had the potential to lead to chaos.”
This feudal political order had the potential to descend into chaos because the ties of loyalty to the central government were very weak indeed. A local knight would, for example, owe allegiance to his lord. That lord might owe allegiance to another lord. There might be two or three more lord-vassal relationships before the chain of allegiance finally made it to the king. This would mean that the knight might only owe allegiance to the king through a chain of 5 (just as an example) intermediary lords. This did not make for a tightly centralized society.
Because the political authority (and loyalty) was not tightly centralized, lords might easily think to look out for their own interests rather than those of the king. If a lord were strong enough, he could create a regional power base that would be able to, in an extreme case, even rival the power of the king. At the very least, there could be multiple local lords, each trying to expand his own power, coming into conflict with one another. This could lead to chaos.
Thus, the central lord-vassal relationship could be dangerous because it led people to be loyal only to the person above them, not to the central government. This decentralized system encouraged lords to pursue their own interests, potentially leading to chaos.