Finally got to this. It's pretty good, and would have been much better had they excised Red's entire explanatory/expository speech. The effect of that speech is to ground the world in a sort of concrete real world logic, to invite us to try to unravel the movie, which simply does not hold up to that level of examination.

The only explanation of the origin of the Tethered necessary is the "once upon a time there was a shadow" fable Red tells when the family is first captured, and the only motive they need is the "to take our time" line she delivers when she's asked what they want. Let the movie operate on the level of intuition and dream. Everyone knows the terror of looking in the mirror and disassociating, their image collapsing to that of a stranger. Everyone has seen their shadow.

As he did with Get Out, Peele is able to summon up some truly indelible images, and leaven the horror with humor without diluting it. But like Get Out, the execution of his ambitious concepts falls short, though Get Out is still a good movie, and Us is just OK.

sidenote: Lupita is terrifying and terrifyingly beautiful.
sidenote 2: I guessed the final twist very early on, which was odd.