The plotting is perhaps a little awkward. At one point, to make certain dates line up, a character is thrown into an SS prison cell for a year. Chapter break, escape, return to Britain, plot continues. I forgive it. Gretel's fate could be viewed as deus ex machina, but since Gretel was a hair's-width beneath godhood for the first two books, it's hard to imagine any other resolution for her. Her arc is undeniably wrenching and, ultimately, satisfying -- in the bleached-bone way of these books -- so no complaints there either.
Ultimately, history is saved. If, afterwards, we look at the real WW2 and think "Whew, reality got off lightly" -- bite your tongue, but you know you thought it -- we'll just have to credit the author with a high talent for descriptive grim.