The most accurate conclusion regarding the vulnerability of the Aztec and Inca Empires in the face of Spanish conquest is: their weapons were not strong enough to defeat the Spanish.
While factors such as internal division, the effects of diseases brought by Europeans, and strategic superiority of Spanish forces also played crucial roles, the technological superiority of the Spanish, particularly in terms of weapons (such as guns and steel swords) and military tactics, was a significant reason for their success against these empires.