This one is difficult for me, because Scourge Unending is not a good book, but the individual parts of the book are better than good, maybe even great. It's not just a reprint of 1st edition's Horrors book, but there are large portions of the text that are just word-for-word identical, and yet, those sections don't actually give you enough information to use their associated Horrors. The fifteen Named Horrors from 1e return, but only the OOC sections that list their stats and suggested adventures. The short fiction that established their flavor is entirely missing. Intermixed with the old Horrors are about half a dozen new Horrors, and they do have in-character fiction to establish them as a presence in the world, but that fiction is in an entirely different chapter, and by the time we actually get to their stats, it's easy to forget which story went with which creature.
The result is a book that can't entirely stand on its own, but is also partially redundant with its necessary companion volume. If you took the two books, spliced them together, removed the duplicate pages, and reorganized Scourge Unending's novel contributions, it would be without a doubt an all-time great monster book. But, realistically, who is going to do that?
The second half of the book, which covers Minor Horrors and Horror Constructs, is rock solid, though. It too reprints 1st edition material, but it does so in its entirety, and the new additions are largely of similar quality (though nothing is going to be quite as inspired as the Dread Iotans). Some of the new guys seem like their primary purpose is to troll the PCs (The Corpse Thief swallows dead bodies and runs away with them, to be delivered intact to a necromancer; the Wasteshadows have no damaging attacks, but sneak into your camp and spoil all your food), but I don't count that as a flaw. Low stakes encounters are, strangely enough, often the ones the PCs get most invested in.
Let's see, what else? I guess I was wrong when I speculated that the new, regrettable Twiceborn canon would never be seen again. However, it's repeated here almost verbatim from Barsaive in Chaos, so I'm just going to act like my stubborn refusal to acknowledge it is merely a single act of willful ignorance.
I did get the feeling that the Horrors are meant to be more of a front-and-center antagonist in 2nd edition. Early Earthdawn books gave the impression that they were a fading threat and that the main antagonists would be your fellow survivors. This book has people speculating that the strange stabilization of Earth's magic level will lead to a second Scourge and that the Horrors might be getting more powerful over time. Neden, king of Throal, has recently recovered from a Horror Mark (a plot also introduced in Barsaive In Chaos), and that's implied to be behind his rash and disastrous decision to attack Thera.
I wish I could say with certainty that this was a case of the new team needlessly simplifying a complex characterization, and not them advancing a hidden plot point they inherited from FASA, but since the licensing deal apparently came with at least two full books of unpublished material, I don't have a lot of confidence one way or another. That said, the conflicts in 2nd edition are feeling a bit starker and less fraught than in 1st edition. I kind of wish Neden's 2nd edition canon was exploring the gay subtext of his relationship with Rozko the Unruly ("he realizes that the state depends on him finding a wife [but]. . . given a choice, Neden would much rather spend time with his childhood friend and fellow navy commander . . .").
(Note: I'm not normally one to indulge in "these two men are close friends, so they must be secretly gay," but when the relationship is brought up in a paragraph that is otherwise about the young man's inability to secure an heir to the throne, that makes me go hmm.)
Sadly, Rozko is nowhere to be seen, except as a bland entry on the "Table of Throal Characters" in Barsaive in Chaos, so our (probable) last view of Neden is as a man trying to be a Good King, but haunted by his close encounter with Pure Evil. It's not a bad idea for an NPC that's likely to wind up the PCs patron (he founded the Hunters of Throal, which gets a whole organizational writeup at the end of this book), but it's a bit more on the nose than I've come to expect from Earthdawn as a whole.
Ukss Contribution: I'm going to go with one of the new Horrors, and it's not even a pity-like. The Horror Cauthrunne is sometimes a giant, troll-sized crow and sometimes a sexy elf dressed in black, and her deal is that she flies around battlefields, singles out a mortally wounded soldier, and gives them supercharged combat abilities so that they'll go forth and make more carrion. As a Horror, she's creating misery for misery's sake, but I like the idea of a magical carrion bird with a pro-carrion philosophy, so I'll probably adapt her into a deadly trickster figure who is only as bad as she is because of the depths of human cruelty.