I read a lot of papers trying to back up exactly that point, mainly for MS and Rheumatoid Arthritis. The problem is that the evidence is all over the place with some studies showing great results and some little gain from CBT/GET - and mostly the research is of poor quality*. That recent MS study is a perfect example: only 26 patients and no control group (results are for pre/post treatment). So while I suspect the point is true, I couldn't find robust evidence one way or another for the effectiveness of CBT/GET in such illnesses. If anyone knows different, please let me know. *it was quite an eye-opener to see just how much rubbish research is out there across a range of illnesses: CFS is by no means unique.