Stephenson had an Authors @Google talk in 2008 [1], which was mostly about his book Anathem. In it he actually addressed what he calls the "Stephenson can't write endings" meme, as he calls it, at 10:54 going on for about three minutes [2]. I think his endings are just fine, myself, and also think that this whole idea that he can't write endings is just a self-perpetuating meme.

[1] https://www.youtube.com/watch?v=lnq-2BJwatE

[2] https://www.youtube.com/watch?v=lnq-2BJwatE&t=10m54s