I've been battling wifi issues (won't connect at work) on both my N5X and my old N5 since updated to Marshmallow. Lots of others with similar issues in the forums. Finally got to the bottom of it, see link below. Appears to be an enterprise server software issue, not a problem with 6.0.
https://code.google.com/p/android/issues/detail?id=188867
https://code.google.com/p/android/issues/detail?id=188867