Just skimmed through this. There's a lot of good content in this, although I do have a few concerns.
I'm not sure why they've described it as "potentially serious". It does not routinely* lead to death or progressive deterioration, but in terms of the impact upon one's life it is very serious indeed.
Multiple sclerosis (MS), for example, was regarded in this way until diagnostic lumbar puncture tests and MRI scans were able to prove its existence
This is absolutely false. It is a common myth (one that also appears in the recent BBC LP article). Whomever came up with this has no knowledge of the history of MS.
There's then a long digression into some of the various abnormalities that have been researched, but it indulges unproven theories a little too much for my liking. I'm not sure that the cortisol findings have been replicated, for instance (lower levels of cortisol could simply reflect our disturbed sleep - usually cortisol is measured at 9 AM when they are at their peak in patients with a normal sleep cycle). The document even mentions the "microclot" theory when it has always been quite obvious that these are probably artefactual. It does acknowledge the unproven nature of these claims but some of them really would have been better omitted.
Sections 2-4 go into detail about PEM, sleep disturbance and cognitive difficulties. These sections are quite good. It's good to see that the centrality of PEM to an ME/CFS diagnosis is acknowledged, that it can be delayed and that returning to baseline can be prolonged.
I'm not sure there's any evidence that ME/CFS and IBS "frequently coexist". It's certainly true that plenty of people with ME have gastro/digestive symptoms but how many of these meet IBS criteria?
The chance of a partial or full recovery is better in children and young people with ME/CFS than in adults.
and
Therefore, it is important that the diagnosis is considered sooner rather than later. The sooner treatment is offered, the greater the chance of getting better.
Not only is there no methodologically robust evidence for these, the idea that there is a greater chance of recovery with "treatment" is just nonsense because, as the document itself acknowledges later (in Chapter 4), "there is no clinically-proven cure or treatment for ME/CFS".
(* edited to add the word "routinely" (see post below)).