Why is it impossible for anyone to accurately describe Fascism and/or Nazism? I've come to believe the fault is with Umberto Eco and his intentionally vague and deceptive description of Fascism combined with the sort of paranoid delusion of far-leftism allowed to run wild.

Eco delibertly defined Mussolinis philosophy in a vague way and at time deliberately (as I see it) mislead the reader. The cornerstone example of this is "corporatism" which is often taken to mean when private businesses run the state.