If by the placebo effect you mean expectancy effects (higher chance of reporting success if one believes one has had a potentially helpful treatment than if one doesn't), then they are a human universal.
MECFS people are likely to be just like other mere mortals, they will be vulnerable to confirmation bias, recall bias, and all the other things that may contribute to expectancy effects.
I agree that much of the placebo effect may be due to expectancy effects, which create a bias in the way patients view and report their symptoms. The following nicely captures what you are saying:
Another study in 2011 on asthma patients found that placebo inhalers had no effect on increasing lung function. But asthma patients nevertheless reported that they felt significantly better after using a “useless” inhaler – a baffling result to say the least.
Source:
The placebo effect: is there something in it after all? | Science | The Guardian
But I am partial to the idea that in addition to expectancy effects, in diseases involving abnormalities in brain dopamine, a placebo may have a direct
physiological action in the brain, based on the fact that positive expectancy raises brain dopamine (which it does: see the PET study below).
This could be why the placebo response in depressed patients is particularly strong: because dopamine itself has antidepressant effects (many antidepressant drugs work on the dopaminergic system), so in the case of depression, placebos may be directly and
physiologically modifying the illness (but to complicate things, there may also be expectancy effects going on as well, which introduces bias in reporting one's symptoms).
In clinical trials, the placebo response rates in depression are 30% to 40%. Ref:
1
In terms of evidence for the dopamine-raising effects of placebos,
this paper says:
The activation of dopaminergic system when giving placebo has been documented by using PET with the D2/D3 receptor–labeling radiotracer 11Craclopride. One study performed on healthy subjects proved that intravenous placebo induced dopamine release at the basal ganglia level. In patients with Parkinson's disease, placebo administration produces dopamine release in both dorsal and ventral striatum, as well as in the limbic system. By providing dopamine at the dorsal striatum level, the alleviation of motor symptoms in Parkinson's disease appears as a logical consequence.
So administration of placebos causes dopamine release in the brain, which logically may lead to improvements in diseases like Parkinson's, where there is dopaminergic dysfunction.
I am however certainly against any "mind over matter" interpretation of the placebo effect, the idea that the mind can somehow magically affect disease outcome. I think this idea is nonsense. If the placebo effect can directly improve some diseases, it will be via tangible physiological mechanisms in the brain, such as the dopaminergic activation that positive expectancy creates.