The idea of shadowsfall being a lost and found collective across all dimensions is fantasic. It makes Shadowsfall the perfect campaign setting to plan a game from 1 to Epic. The exspansive world Shadowsfall can be allows any DM to create entire millenia of backstory to add endless flavor or twist to any campaign.
I like the new races; but Shadowsfall has nothing in the way of new classes, which is very odd for a shadow plane of everything forgotten or lost over the known existence of time. The Wanderer Race is especially intruiging. A fallen angel, Ill-suited to being a paladin or cleric; that can wield a holy weapon no matter what their actual alignment is. Seems like it could make a good reason to play an evil rogue sneak attacking with a holy avenger. Just because you can. Or perhaps a two weapon fighting Warrior with both a good and evil artifact sword, both with their own personalities and psychic voices so they can argue throughout the entire campaign.
I see no true flaws with this book or the campaign setting. It's more vast and open ended than any other world, which for any other setting would make it seem incomplete; but with Shadowsfall it sparks the imagination into the possibilities of the forgotten past and actually encourages DM's to spend months {or even years} creating a unique version of shadowsfall, with special qualities from forgotten pasts that hint at forgotten wars covered up by powerful magic and ancient devices with unknown purposes from unknown planes of existence.
The possibilities with Shadowsfall are truly endless, and as a DM who tends to create campaigns from scrath {or even creat new games altogether} Its nice to finally have a concrete setting with a very changable world.
As a side note to players; look up the sword and socery: relics and rituals core book. The Penumbral Lord Prestige class is an excellent choice for spellcasters in Shadowsfall, if your DM will let you use it.