Madame Braddon was so prolific in her time, but so forgotten now that it's hard to find all of her books. Many though are available through Project Gutenberg and Google Books and I'm very happy to have them. In terms of writing, themes and structure they're not much different from Dickens or Collins. Dickens has more quirky and hard-hitting characterizations, Collins more complex plots, but for sheer atmosphere and page-turningness, I think Braddon beats them both. And that's why I enjoy her books. Even separated by 150 years and an ocean, I find them very easy to read and a lot of fun.
As with Lady Audley's Secret, the "secret" isn't one to the reader and the joy of the story is to watch the other characters figure it out and bring the miscreant to justice. Henry Dunbar is a bit more satisfying in the sense that the villain is more villainous than Lady Audley was. As a modern reader though, I wish he had more direct "screen time". That is that I wish I had more of his internal thoughts and motivations directly from him. As it is, the writer shields his crime continuously throughout the book; hoping, I guess, that the reader didn't figure out the twist. It's pretty obvious though and so not acknowledging that she might have smart readers is a bit annoying and I think including more of Henry's internal struggles would have added a new dimension to the tale.
For the most part, Braddon does a great job inserting little hooks and predictors into the story to compel you to keep reading. Things people do, say and remarks the storyteller makes directly are all part of how she keeps the plot on the boil.