I have to be honest and say, I think she's a very good writer, but I don't really enjoy her books. Dd1 hated them, dd2 loved them.
They feel rather like misery lit for children, or some of the Catherine Cookson books. You can guarantee that the main child has significant issues, a good number of the adults have issues, and it's not necessarily all resolved by the end.
The main child is normally a girl (can't think of any where they aren't but I might be wrong) but is also rather gobby and rarely would I think they're someone I'd have liked to be good friends with. They might "get together" with a boy they initially didn't like who's normally either geeky or fat.
She's the only author I've removed books from dd2. I removed one of the "Girls in Love" series where I think JW crossed the thin line between informing readers about anorexia and giving them a manual, or even glorifying it. I'm a bit sensitive on that because I've lost a friend to anorexia, but with dd2 who is definitely impressionable and a bit of a tendency for going overboard on things, I could see it definitely giving her ideas.
The other I removed was "Love Lessons" as mentioned before. I was astonished to find it was published in 2005. I thought it would be 80s at the latest. I'm sure she wishes she hadn't written it, but it is still published today. I would have expected one written at that time would have been far more about the teacher as a groomer, rather than the child leading him on, which is definitely the impression got from the book.
Some of the books I like. "Double Act" is good. "The Lottie Project" also has a lot for it and there's others of those type which to me work well. I think they're the only two I'd pick up again though.
I mostly like the first "Hetty Feather". I'm not very keen on Hetty Feather as a character; at times it felt like she'd picked Tracy Beaker up and dumped her in a historic setting. Thing is, for me, whereas Tracy could get away with her behaviour in the modern world because people would have tried to understand her, Hetty wouldn't have been given that leeway. I feel the same about Harry Potter in the first book when he answers Snape back-he has been bullied and disliked all his life, he would not have the confidence to do that at that point.
The later Hetty Feathers feel like they're stretching a very long point, to a point where she feels a bit Mary-Sue to me. She wants it, she gets it, even if the chance of a young girl getting her own way in that era is probably less than unlikely.
And the last book featuring HF that dd got, ("Diamond"?) felt like she lost confidence in Diamond's character halfway through. I liked Diamond (if that was her name). She was a quiet unassuming child, unlike most of JW's characters, but she was getting things done in her own way. The book was going really well, when Hetty Feather appears. And from then on, Diamond is really a backstage character to Hetty; even when they escape, it's Hetty who does it all while Diamond passively does as she's told. Its Diamond's book-she should have been the one to plot their escape!
I was trying to think if the books I read at that sort of age were similar. I know I read a lot of teen-anxy type books, but my memory is that things were generally resolved by the end.
I think I'd describe reading one of her books as going into a cold rough sea. You don't really enjoy it but you may come out feeling a sense of accomplishment.
And tbf to her, what I want from a book is more the feeling of dozing happily in the warm sun. Comfortable and peaceful. So I don't think I'm the chosen audience!