I think the reason ISIS is so hard to understand and pin down is that the ISIS we know is a media product. It's a propaganda prop, created by a group behind the public face of ISIS, with objectives and methods that differ from what they publicly claim. For instance, there are clear indications that the military leaders of ISIS are ex-Baath generals from Hussein's Republican Guard, and those people are not by any means religious fanatics. They're pragmatic military commanders who used to be part of a secular organisation.

Now, usually it'd be easy to see through the propaganda, except the propaganda is useful to both ISIS and its detractors. To ISIS, the image of the oncoming storm that will reduce its enemies to a medieval Califate works because it strikes fear in their enemies and serves as a recruiting tool in the West for the deranged and social rejects; and meanwhile, Western governments can use the exact same propaganda about ISIS to rattle sabers and rally the Conservative base in efforts to control immigration and finance wars abroad.

In short, the truth is out there, but it benefits neither ISIS nor its enemies.