[0] In particular, they did not (and physically could not) have launched a broad enough manufacturing capability for the remnants of humanity to have had a realistic chance of long-term survival. The practicality of the Seveneves themselves did was also at best dubious from a biological and other standpoints.
Stephenson does seem to have a history of fudging a bit of the realism to drive the plot, like anybody, but he seems to do it in a way that says "wink wink, I know this couldn't happen but work with me people ;)"
If you like Stephenson, read this, but if you've never read one of his books before, I would not recommend this as a first.
That's what I hated about the second half. It's like inverse Ringworld - you know how the world was built, and why, but you don't know much about the inhabitants, and your tour guides aren't very interesting, either.
I liked the first half - it was interesting to read about them solving problems, even though the problems - and sometimes the solutions - were atrocious.
I find the real world has enough of that - I want my fiction to be more escapist and fun and uplifting.