I think the terrible endings are for the books that he has to meet deadlines on. Both Seveneves and Termination Shock suffered, in my opinion, from premature endings.
Anathem is the one book of his I've read that I thought had a really satisfying ending. It is also by far my favorite book of his. I wish he would revisit that universe. With Seveneves, it seemed to me that the quality of writing and pacing of the story changed somewhat abruptly near the end.
Did you feel that way about Anthem or Seveneves? With Anthem, I felt like he had just finished laying the groundwork for an impressive feat of unique world building, which felt a shame to abandon at the end of the book, even though I enjoyed the ending. With Seveneves, I felt the book ended shortly after part 2 finally started to pick up pace, with several new sets of characters, and their dynamics, being introduced right before the end.
The world building is why I can re-read Anathem endlessly. The detail is just wonderful. Every time I finish it I get a feeling of grief knowing that it'll likely never be expanded on.
