I've written fairly extensively here about my own experiences with saunas, a few times. Advanced search posts by me with "sauna" in them should find them (one day maybe I'll index some of this stuff better).
Anyway I'm one of those who found FIR saunas very helpful and although I was doing a lot of things at the same time, I consider the FIR sauna to have been crucial in my own 'recovery' (I gradually regained most of my function during those 2 years or so but I'm still left with immune abnormalities that I have to manage in order to stave off relapse, and I still do frequently begin to relapse when I'm unable to avoid situations of mold or perfumes etc).
Key things I was advised about the process, from multiple sources including Dr Myhill's site, a doctor affiliated with her, and Allergy UK.
First (as confirmed by my experiences) anything less than your own personal FIR sauna is no good. They are actually not too expensive. Public saunas, I found, were disastrous. One explanation given was: when our pores are opened, we are vulnerable to toxic exposure from (eg) the makeup and anti-perspirants used by other people who have used that sauna. We're wandering into a chemical stew and opening our pores. The other angle is of course that intense saunas put a lot of pressure on the body, whereas FIR saunas, carefully managed, are less intense but more targeted for detox purposes.
I too have heard that FIR saunas don't really sweat out toxins as claimed, however the people making that argument always seem to be those who also claim that the very idea of chemical toxicity being a health problem is quackery, and they don't believe any of this stuff - so I decided to see for myself, and having done so, I now take the "debunking" of these sort of treatments with a large pinch of salt.
I started to see results after several unproductive sessions, maybe it kicked in on the fourth or fifth - and I could feel the difference in my response as it was happening. I could also smell that my sweat was different on these occasions. I did take a kind of cool plunge bath to wash off the sweat, mainly because I was worried about infecting things with the sweat and I wanted to wash it away rather than "reinfecting" myself. This was OK for me, but I think people who are more physically challenged (eg with more severe orthostatic intolerance than my own) would not be able to this, as many have stated is the case.
Finally, the crucial point that I haven't seen emphasised: while detoxing one is also losing lots of valuable minerals as well as the toxins, and of course losing fluids too. So it's crucial to replace the lost fluids (with lots of filtered water), to have a targeted supplementation regime, and to replace salts etc, and this supplementation regime should also be tailored to the individual's vitamin deficiencies etc. Constructing this supplementation regime is complex as it varies from individual to individual and some of the tests are fairly expensive - and of course very few doctors understand the immune biochemistry involved so it's hard to find one and they aren't cheap (but very good value in my experience).
So in summary: the whole thing will likely often not be effective without a program directed by the right kind of physician, but in my experience, I believe it was a valuable part of my own recovery. Based on what many people have said about their bad experiences with this and other treatments, I think it's most likely that the most severely affected CCC ME patients will not be able to tolerate this treatment sufficiently to gain benefit from it, or at least, they would require extremely good supervision and a lot of supplementation in order to do so. Until I hear of someone who has recovered from severe ME (rather than "MCS") with FIR+supplement programmes, then I have to consider that it's only a proven treatment for some milder cases - but a lot of us have benefited from it, that's for sure. It is certainly one of those things that has to be done in the right way and which requires some perseverance to get it working.
Oh and finally: any benefits I got from it were certainly not spiritual or meditational IMO. I used to watch films while detoxing, and there was nothing particularly spiritual about my experience. Speaking as someone who's had quite a lot of spiritual experience, I think I'd have recognised if that were a factor for me with this treatment.