Problems arise owing to the entanglement of the existential and essential, structural and function, static and dynamic, as well as of the cognitive, volitional, and affective, as psychological aspects of being human.
It's called "the human condition." Assuming that humans are perfectly "rational," or even that they can be perfectly rational, is contradicted by both experience and psychology.
Humans are not only bounded rationally but also cognitively, and humans are also differentiated by different types of thinking and feeling.
The assumption of a homogenous "rational agent" is more restrictive than the reality. This doesn't mean that it is useless for modeling. Just that such models are limited and extending them beyond the scope of the assumptions leads to over-generalization.
