Mortal Remains is useful, but far from brilliant.
The Appendix, updating Hunter the Vigil to conform with The God-Machine Chronicle rule update, is really the prime reason to buy this book. There's a chapter providing a variety of Dread Powers, which can also come in handy when depicting one of the monster types in the book, or when creating a new one. But the bulk of the book is a review of Promethen, Changeling, Geist, Mummy and Demon through the Hunter lens. And it is here where the book feels weak.
The Promethean feels the most fleshed out. The chapter spends sometime about the challenges of identifying a Promethean and curiously focuses on Demiurges as the catalyst of confrontation between Hunters and Prometheans. The rest of the material is informative but uninspired.
Changeling similarly feels very "by the numbers" with various compacts and conspiracies adopting "fey" terminology. I found this disappointing, since even the Changeling core book played out the similarities to modern "Alien Abduction" stories. And Hunters trying to identify Changelings along those lines could have been far more intriguing. the idea of Fetches almost invites dis-information about these creatures and their motives. Rather, the chapter presents little to no confusion and misidentification about what changelings are, with some Hunter organizations even recognizing the True Fae as the greater threat.
I feel a bit ill-equipped to comment about the Geist chapter, since this is the one core book I haven't read. The chapter seem to struggle to present Sin-Eaters as something inhuman.
Mummy was a relatively strong chapter. It focused mainly on Hunter interaction with cults in service of the mummies. The chapter focused on the theme of "the unknown" to great effect, depicting Mummies as a threat Hunters only encounter after tearing down a vast conspiracy, if all. The Aegis Kai Doru's response to Mummies is justifiably much more detailed then any of the other organisations.
The Demon Chapter suffers from obvious lack of space. It tries to tackle both the Vice driven demons depicted in Inferno and the Bio-mechanical quantum state Demons and Angels of Demon the Descent. Again, like changeling, Hunters seem to recognize Demons as Demons despite the fact that they differ significantly from popular myths. The book fails to address how difficult it is to identify and corner the Unchained and only pays lip-service to the idea that these entities can easily infiltrate or found any of the Hunter Conspiracies or Compacts. The two Demon specific conspiracies provide some interesting ideas.
In my opinion the problem is not the limited word count but the lack of focus. More could have been done with the same space. If each chapter would have chosen a singular theme to focus on (like the Mummy chapter) and chosen three to five Conspiracies to flesh out each chapter (rather then all of them every chapter), the book would have benefited from it. Beyond these technical issues, the writing is just not as inspiering as I would have liked. And the ideas presented are quite banal.
By no means a bad resource. But one that is much more useful then inspiring.
[3 of 5 Stars!]