IMO, the important question about a film is "Is it a good film?" not "does it stay true to the source material". Because at the end of the day its more important for the film to work on its own terms.
The example I liked to use is Hitch****'s Young and Innocent. Based on the book "A Shilling for Candles" by Josephine Tey, the movie drops the Inspector character, changes the plot from a Whodunnit to a Double Chase and is generally a completely different story. So as an adaption its not faithful, but as a film on its own its one of Hitch's stronger British films.
At the end of the day, I think being a good film will ultimately triumph over being a faithful adaption but a weak film because of it.
As I understand it
*backs away slowly*
I liked The Uninvited too; although I knew the twists because I'd seen the original Korean film, A Tale of Two Sisters.