Like Grace points out, subjective experience is a critical defender against radical illusionism, and personally I'm partial to radical empiricism: take subjective data seriously.
The illusion is imo not about C as such, but how C presents itself to us (in terms of C).
The data for how C/Mind self-organises, the process of 'moving' (inference/flow/dynamics) and the space in which they move, should have some invariant motif across individuals/systems.