Note that in the given source
https://www.cfids-me.org/ramsay86.htm, "emotional lability" is mentioned as one of the cardinal features
of the cerebral dysfunction in ME, not a cardinal feature of ME itself.
In the first edition of Melvin Ramsay's book, 1986, a source for this document*, this doesn't even get such prominency as diagnostic part of ME.
Ramsay is discussing how to make a diagnosis, and in that light discusses the three main features of the illness that in combination make it easier to diagnose ME, what he calls the 'diagnostic triad':
- extreme fatigue/dominant and persistent muscle fatiguability, that is negatively impacted by exercise and has a delayed recovery
- susceptibility to cold/climactic change (which he relates to impaired circulation)
- cerebral dysfunction
Therefore Ramsay advises to examine ME patients after exercise, and do "close interrogation" about the other two points. (Makes sense no? Basically PEM, not being able to handle cold and climactic change well, and cognitive symptoms, and have all three present for a positive diagnosis.)
Emotional liability is named as commonly occuring among a bunch of other things falling under cerebral dysfunction. In a repeat of what according to him constitutes ME further on in the book, he doesn't mention it again.
(Emotional liability is mentioned in the book on multiple occasions, but as part of reporting on what was said about patients in earlier outbreaks in the 30s, 50s and early 60s. It might very well just be a reflection of male physiscans looking at a patient population that was largely female. It's why "hysteria" is mentioned a lot too, which is saying more about medical mysogyny than actual patient observation. )
Between gendered prejudice and patients having the stress of an intense, disabling illness, I'm sure "emotional liability" has been "observed" a lot. But I doubt it should carry much weight beyond showing that chronically ill patients need adequate support for dealing with their illness if it becomes too heavy a burden. (Before BPS takeover it was accepted as normal that about 30% of people with a chronic illness were having a depression as a logical result of them having to live with it.) And if you look at the pieces of text where Ramsay sets out painting a picture of ME, it seems he didn't give it much weight either, beyond that it could occur.
*(I don't have the 1988 second edition)