I'm thinking of starting up a 40k RPG PbP and I have interest in both Only War and Dark Heresy 2e, so I am testing the waters of each, specifically their published adventures.

I am curious about the Forgotten Gods adventure book. Mainly, I want to know who the primary antagonists are (Chaos, Genestealers, etc.) and what Ordo would be most appropriate for the adventure. From what I can gather on the Blurb on the FFG page it seems like the adventure is somewhat tailored toward the Ordo Xenos. Is that correct? If so, how difficult would it be to tailor it toward other Ordos?

I'd say it is more a Ordo Hereticus, but you can play it with all Ordos.

The hook is that there seems to be a war between a xenos artifact smuggling ring and a cult. So you can investigate as Ordo Xenos, because of the artifacts, with Ordo Hereticus, because of the cult and smugglers or with Ordo Malleus, because the artifacts have a daemonic warp taint.

Should be pointed out that Forgotten Gods is intended to be the 3rd part of a series, playing off both the module in the core rulebook (Dark Pursuits), and the GM Screen (Desolation of the Dead). Obviously you could run Forgotten Gods without running either of those, but there are story hooks in Forgotten Gods that wouldn't make as much sense.

Its definitely written to be pretty "Ordo Neutral," as there are components of the investigation that would pique the interest of an inquisitor of any ordo (besides, its not like one inquisitor would just let heresy occur under their gaze just because its of the wrong ordo).

Antagonist wise, it is indeed mainly xeno artifact smugglers, cultists (who believe they are worshiping an alien god). It does provide opportunities for the acolytes to appear to be on the wrong side of local law enforcement. Dark Pursuits and Desolation of the Dead are similar, same hive as the setting. Dark Pursuits is definitely far more related in a broader sense, similar smuggling antagonists, whereas Desolation of the Dead introduces the the NPCs who brief the party at the start of Forgotten Gods. Desolation of the Dead also does a far better job presenting the players with the visual setting of the Desoleum underhive (where a chunk of act 1 of Forgotten Gods occurs).

I gotta say, having read it, I'm kinda iffy on the book. By all rights, its a fine module (and its certainly nice to not have to manage the details of the game solely on my own), but as a warning, much of it is either a chase or at least a tense "race against time." Little to be had as far as a long term investigation or even real downtime (except I guess a chunk of act 2). While obviously a GM can reign in a stray party, it feels that if the party loses track of their objective (by say, failing to notice the tracks, or latching onto the wrong detail), then it'll feel like a stretch when they arrive at their destination with what only seems like minutes to spare. Healing/group downtime feels like a real problem here.

The main big and awsome hooks are in Dark Pursuits for Forgotten Gods. I'm currently running Forgotten Gods after Dark Pursuits due to my group vetoing the idea of more hive adventures, and I think they mesh very well together. You're are basically tracking down the group that sold the orb and it all turns out to be this massive conspiracy . Other than who the callers are you don't get much from Desolation of the Dead.

I've always felt this to be a Hereticus campaign, but it's not hard to justify other ordos taking an interest. It's really just a good introductory campaign. You get a lot of the elements of the 40k universe.