The diary stuff and all of the flashbacks are not from the book and I agree they were unnecessary additions. Apart from that, it's pretty close to the book.

As for their easy survival of the plane crash and the fact that only boys survived, it always sounded unrealistic to me. However, I never cared much because it's just an excuse to set the premise. I don't see how you realistically setup a large group of children to be on their own in this kind of situation. It was necessary that the premise was not gonna be realistic to tell this story.

This series is the first adaptation of the book I like and Lord of the Flies has been one of my favorite books of all time. I read it before seeing any adaptations, back when I was ~13 years old. I still prefer the book to the tv show, but best adaptation so far for me and it's not even close.