Problem was the marketing, not the movie SPOILERS
People were expecting the wrong movie.
This wasn't a cop thriller, a mystery, or a horror movie. It was simply a drama looking at how people fell for all that SRA crap back during the Satanic panic era. The "twist" wasn't lame because there was no twist. It made clear from the beginning that the accusations were bogus (by referencing that it was inspired by other Satanic panic cases).
Throughout the "investigation" we're shown just how such mistakes are made: by investigators asking leading questions during interviews, by mental health professionals practicing half-baked techniques, and by all involved letting their imaginations get the better of them, leading to widespread hysteria.
Many of the things depicted in the movie actually happened in the late 80s and early 90s. There was a book (later discredited) by a so-called survivor of SRA that cops really used in investigations. There were TV specials like the ones Hawke's character was watching that helped spread the panic. And there were kids who just made up stuff...and parents who pled5 guilty to sex crimes they never committed.
I think the problem is that people went into the movie with mistaken expectations because of the way it's been described. If you were expecting a thriller or horror movie and didn't remember much about Satanic panic, then yeah, you'd be disappointed.
Think of this more like a Lifetime movie, but with much better writing, actors, and production values.