OK. I just spotted the announcement to modify config.xml use an older version of phonegap by adding this line:
<preference name="phonegap-version" value="cli-5.2.0" />
This now allows Windows to build an .xap for download. Note it previously built .appx.
The iOS is still failing to build. The contents of the log are just "Build Date: 2016-08-23 06:00:26 -0700" which is not giving much to go on.