Fallen Suns is a large adventure, with interesting hooks, and with dangerous opponents (I've counted a few ways so far that an unwitting GM could pull a total party kill given the forces in a couple of the sections), but it's not perfect.
The threat seems credible, as does your rival. Your explorers will interact with a few characters worth keeping around, and there are a lot of Eldar names and stats that will be very useful. The information on the Craftworld is good too, and could easily be reused depending on how much your explorers like (or dislike) dealing with Eldar. While not completely generic, Fallen Suns at least tosses some good motivations and forces at you for different Eldar groups, which could be useful if your group skips over certain parts.
My biggest problem (and the reason this didn't get a 5/5), is that the writers seem to have no sense of scale and the proofreading is typical of FFG products (not bad, but confusing if you aren't paying really close attention). In one section, they never do explain the Chaos forces actually arrayed against you, and flip back and forth between calling lead ships Cruisers and Heavy Raiders (big difference if your Explorers have a Grand Cruiser!). Given the stats for Chaos ships at the back of the adventure, the GM is going to have to modify the end to make parts of it a credible threat to the explorers. Not a huge dealbreaker, but I'm a detail type that has a serious issue with those sorts of things, especially in a book like this.
They also switch between calling the main Eldar ship a Cruiser or a Battleship (less of a deal, but its obviously a Battleship), and toss a couple important challenge rolls at your explorers that, if they fail the roll, stops the adventure cold in its tracks at critical moments. Modifiable, sure, but I'm of the opinion that the players should always be able to move forward, even if they mess up - it might be harder for them, but shouldn't bring them to a dead stop.
I'm planning to run this, since it sounds interesting, we've already run Frozen Reaches (a good adventure), and my game has a pretty heavy Eldar focus; but a GM is definitely going to have to read this and modify it a bit to make it work.
Threat to Explorers: 5/5
Scale of the Universe: 2/5