Just to pick up on this one point, actually it can be better to know. I've lived with mental distress all my life, due to events that happened in childhood.
But when I was finally informed of a (roughly) 12 - 16 week period of extreme neglect as a newborn in between biological and adoptive parents then finally some of that distress made sense. The damage can't be undone but at last I know why the distress seems to go so deep. It was a very big piece of the jigsaw puzzle and it has helped a great deal to know what happened. It stopped me feeling there was something intrinsically wrong in me.
It also explained certain physical skin issues that according to the biological parent I wasn't born with, but adoptive parents said were present when they got me, and the issues are most likely to be the direct and lifelong aftereffects of a physical experience. For me, finding these things out has been a good thing and oddly enough, mentally freeing even though the skin problems and other distress will remain.
It's incredibly difficult to know what to say or not say for the best, and also at what point anything should be said.