The problem is that ME/CFS is similar in presentation to other conditions such as for example certain kinds of depression. The difference is that exercise is thought to help with those conditions and exacerbates ME/CFS. Thus there are two different groups of people, one who you are helping by encouraging them to exercise and one who you are harming. If you can't tell them apart the system is bound to hurt someone.
It's proven to be the case. A repeat cardiopulmonary exercise (2-day CPET) is currently the only widely replicated biomarker for ME/CFS. Only ME/CFS shows a reduction in performance and a lowering of ventilatory threshold on the second day. Anyone else (sedentary, cancer, heart or lung disease, MS, depression) will show an improvement. [1]
A study this year showed a striking effect looking at the metabolites in urine after exercising female ME patients vs sedentary healthy controls. The ME patients just did not excrete metabolites as the healthy controls did. [2]
"This indicates that ME/CFS patients have a general metabolic dysregulation that is part of their exercise intolerance and PEM in which altered metabolic excretion is a contributing factor."
There's also a very simple clinical discriminator between ME/CFS and depression. If you ask a depressed person what they would do if they were suddenly cured, their answer will be "Not sure, I don't know, I can't think of anything, nothing really." If you ask someone with ME/CFS the same question, the response would be "Go to the shops, drive my car, walk on the beach, see my friends... etc".
Hence it is important to make a distinction between the two, so you give proper treatment.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7988339/
First problem, I do not have ME/CFS, let's use the Maes et al (2012) scale. 0-5 scale, 5 being the worst.
"1 means mild exacerbations of fatigue/pain/neurocognitive symptoms following exercise (either cognitive or physical)" Yep, after a longer, harder than normal bike ride I'm quite fatigued, in quite a bit of pain (bad back, knee, shoulder), and can't think particularly well.
"2 means moderate exacerbations of symptoms following exercise; 3 means severe, incapacitating exacerbations lasting less than 24 hours;" Accurately describes what happens when I do HIIT style intervals that exceed my work capacity, or what it's extremely hot outside. Dreadfully exhausted, usually have to nap, or lay on the couch, every muscle hurts, extremely sore, difficult & painful to move for 24-72 hours.
I'm not saying ME/CFS does not exist, this is just a terrible indicator.
The worst part is making it so specific obscures the large body of existing research that would provide tools for treatment protocols here, eg work capacity in athletes.
I know you said you don't have CFS, but trust me - the fatigue we suffer after PEM is nothing like "normal" body fatigue after exercise.
I think we'll find a diagnostic test eventually. At least, I hope we do.
Cochrane systemic review, "exercise therapy for chronic fatigue syndrome", 8 studies, 1518 participants
"Investigators compared exercise therapy with 'passive' control in eight trials, which enrolled 971 participants. Seven studies consistently showed a reduction in fatigue following exercise therapy at end of treatment, even though the fatigue scales used different scoring systems ... Serious adverse reactions were rare in both groups (RR 0.99, 95% CI 0.14 to 6.97; one study, 319 participants; moderate‐quality evidence)"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6419524/
Fairly plausible, mitchondrial biogensis is an adaption to exercise, it may be impaired in ME/CFS patients, but not totally inhibited, and more mitochondria may lessen the severity of the pathogenesis of ME/CFS.
My experience (in California) has been that most doctors have been willing to assume I am being truthful, but a significant fraction seem to want to spend their time and energy only on conditions that the patient cannot be deceiving them about.