I don't know anyone IRL with persistent MECFS who cured it with brain retraining or any psychological approach, but I do think mind-body stuff helps some people with chronic pain issues. MECFS is different. Maybe it's helpful for people suffering from fatigue - not really sure. But that's really not the same thing.
But there's no understanding of the subject IMO, because it's all based on pseudoscience. None of it is properly validated, and the studies are a joke. They only promise that brain retraining helps for diseases with no biomarkers - not a single biomarker driven disease is included.
In addition, I tried it (Gupta) and it did almost nothing for me. I found repeating words somewhat pleasant (like a mantra), but beyond that - zero effect on any of my symptoms. And the hours of videos they make you watch about recovery stories (which feels like brainwashing) have a bunch of terrified looking people with no energy in their voice who look like they're suffering from serious anxiety issues. They couldn't even find people for the videos that looked healthy?
Not a huge fan of brain retraining, in case I wasn't clear.