Question is was McMurphy even real. Or at least is he actually the person the Chief depicts.

The book was about the power of an institution to fuck ppl up. It's told by Chief Bromden who rebels because of (or at least after) what he sees happen to McMurphy.

Bromden was clearly ill. He's not a reliable narrator.

Different case with the film in this case the story is face value and Mac is in the Ward to avoid real jail. He is ill and a pathological anti-authoritarian figure but he's not suitable for or in need of a psychiatric hospital.