Once the various entities willing to do scientific studies of hypnosis can agree on a consistent, specific definition of what a "hypnosis state" is, and what is not a "hypnosis state", and then agree on a standardized procedure for both inducing hypnosis and objectively determining whether a patient is properly in that state or not, I'll start considering hypnosis studies useful. Until then, it may as well be a dozen different people studying a dozen different but similar-appearing things, and calling it all proof of the same thing.
And I'm a little leery of "brain activity" as a proxy for any particular, specific mental state.