It's almost uncanny how "middle" the middle of trilogies can be. There's probably a science, or least an understanding, of this, but as a consumer of media I can't help but notice it.
And so it is here. Shadows of Self is a well padded story (that is to say not the most efficient), but does do its job of expanding not just the second Era of Scadrial, but also the previous trilogy - there's even what I suspect are breadcrumbs that lead to an insight to the Cosmere as a whole grand plan.
So exciting enough stuff, but the book still manages to underwhelm with its timepassing approach. Yes, there are twists and retcons, and yes, the story is decent enough. But overall the empty calories become just too much to bear. That being said, I suspect reading the next part in the series back to back with this one will serve to enhance both. We'll see.