I actually explained in further depth in the recent EMDR wiki page that is currently under editing, [DLMURL]
https://www.myptsd.com/threads/eye-movement-desensitization-and-reprocessing-emdr.86472/ about this exact thing, and you will even find the referenced location from Dr Shapiro's own book that outlines there is a possibility for brain impairment in the neurological processing, however; I will say, its a very slight chance. The biology heading outlines the biological aspects of it, though you should read it all to fully understand the EMDR process, as the page linked works as one, not each section.
The high risk categories for this is combat and complex based PTSD, because both trauma types are proven extremely resistant to therapy and pharmacology due to the severity of the trauma type. Not all... but within those two groups, there are high percentages that have either amnesic effects to their trauma, which means dissociated. When you read the process linked above, and understand how EMDR works, then you can understand how suddenly mild brain impairment could be a risk factor.
The page says it best IMO. Overall, I would actually recommend EMDR nowadays for most people, as the majority of PTSD patients aren't in the severe / combat / complex trauma types, they are majority in one off traumatic events and recover primarily with time, though trauma therapy intervention rapidly speeds up that process.
Those in the combat and complex trauma types, should first educate themselves extremely well, provoke themselves somewhat with attempts at exposure and such to get a feel for what will come with trauma therapy, and try and nudge anything really bad within their brain out under such circumstances... then give it a shot.
Quite honestly, the physicians themselves should be finding such people at high risk within the screening interviews and rejecting them, however; some are not fully qualified correctly, have only read the book and taken the book course with certificate, which is not an EMDR Institute endorsement of accreditation, which some use as such. The other side, is that some just don't put the patients health first, and instead see it as a challenge, which with other therapy types is ok, but should not be done with EMDR.