The issue with NAC in CFS does indeed involve mercury. It has been found by Aposhian et al. that NAC can move mercury into the brain in rats. Dr. David Quig of Doctors Data Lab has recommended limiting the dosage to 300 mg per day if there is thought to be a significant body burden of mercury, for this reason. Dr. Cheney has reported that he has heard of two people with CFS who committed suicide after taking large dosages of NAC. I don't know how large the dosages were, or why they committed suicide, but perhaps mercury neurotoxicity is a suspect.
Freddd reported that taking NAC and other glutathione precursors set him back considerably, but I think it's important to note that freddd has reported that he has an inborn error of metabolism that involves the intracellular B12 processing enzymes. Apparently, his cells are unable to utilize vitamin B12 once it has combined with glutathione, which is a normal step for most people. So freddd's experience with NAC may not represent what would happen for most PWCs.
On the other hand, some PWCs have reported benefit from taking a combination of NAC and the other two amino acids that make up glutathione, thus supporting the liver's ability to synthesize it. This combination was sold on the internet for several years, and perhaps it still is.
So that's as much as I know about it.