CFS, definitely. ME might be like that, but it is not clear. The clearest example you can use to show this problem is depression. Depression, fatigue and pain are symptoms of disease, not diseases. Any diagnosis based mainly on one of these three, or any combination of these three, is problematic. Depression can be caused by many things, and its a category mistake to think its a discrete diagnostic entity. One can diagnose the symptom, but cannot diagnose the disorder or disease without making this error.
We also need to be careful about confusing a kludged disease category with any underlying disorder. ME may be many disorders, or one, but if its kludged with other diseases to create a bogus category it is not evidence that ME is not a discrete disease category, or several.
A similar mistake is made in medicine when they find nothing wrong with us using standard tests designed for other problems. Its also a problem when they have no test for ME and yet conclude there is nothing wrong with patients. They have to pay attention to the science, and most don't know the latest science, and test for what is testable. The first test that is proven to be useful dates from 1940, and that is the tilt table test. We can show specific problems even if we cannot diagnose ME using a test. So anyone making these claims that nothing can be found is about 77 years out of date. To be fair though, detecting OI in ME was not validated till 1995. But even if you use this date then doctors are still 22 years out of date.