I'll just add my voice to this as a counterpoint. As I was reading, I never had the thought that this would be such an unpopular read, and I came away with a wildly different opinion from the majority (that has posted thus far). I thought very highly of the book altogether, though the second half seemed to drone on with not much that I found interesting.
The idea of this being a story with no relevance to the rest of the Heresy is off base. If nothing else, the epilogue is one giant 'To Be Continued,' with a BBEG (as the OP termed it) let losse and needing to be accounted for. With very little mental arithmetic, you can see how this would clear up one of the series' major plot holes for the future. That alone is a (minor) progression of the storyline, and probably the most important thing to take from the book. If dinosaur fights are your major takeaway, you may have been focusing on the wrong things.
The part I was most struck by, however, was the opening half of the story, before the cultists are introduced and the legions land on Pythos for the second time. This is the portion of the story that focused on the fallout of one of the larger storylines of the Heresy - the death of Ferrus Manus and what it means to the surviving Iron Hands. It really hadn't been addressed to any meaningful degree before Pythos. The Sevatar short (Prince of Crows) covered the chaos of the almost-death of Curze and how the Night Lords coped. And Riven scraped the surface of the Hands post-Isstvan trauma for the Hands. This book goes so much further in depth, including the effects on the serfs and human descendants of Medusa. If nothing else is of interest, at the very least it is worthwhile to see where the legion's rage comes from and how they are able to use it to guide them. The most important and beautiful passages of the book deal directly with this idea and (IMO) are as impressive as any passages that I've seen in the Heresy to this point.