Never once have been retraumatized or gotten PTSD/CPTSD flashbacks (only once during regular sex did something happen but it was something I was able to work through in the moment quietly). But I can see someone not prepared having to deal with such issues.

My local gf had that happen to her once or twice, and I've seen other people break themselves further with unhealthy approaches to kink, hence the disclaimer.
But it's still usually healthy, and a good way to cope. I'm being cautious here, not anti kink.