One thing that neither the book nor the movie addresses is the fact that at the end of the story, Hannah is, in essence, a Holocaust survivor. Many people suffered lifelong trauma from that experience, and now that Hannah "remembers" the Holocaust, the same could very well end up being true for her. It is possible, given the way that it happened, that some element of the transition will shield her from the worst of the memories and that what she remembers will be only the broad strokes and not the traumatic day-by-day parts (in the same way that Hannah in the book forgot about her modern life for much of the story), but there's nothing in either version to definitively indicate that this is the case.
What's worse is that if Hannah does suffer ill effects, actually getting proper help to treat the issue is likely to be difficult if not impossible because no one's going to believe that someone Hannah's age actually experienced the Holocaust. A mental health professional would presumably figure that it was a delusion or hallucination and try to treat her on that basis, which would be unhelpful at best for treating what would actually be trauma from a very real experience.
