Looking at the documented list of installed packages from https://github.com/travis-ci/packer-templates/blob/e193d273e62fd7eaaf8528d5e5643ceb8c15464f/packer-assets/windows-server-2016-ci-onion-packages.txt and comparing to the output of
choco list -l reveals a few differences. There are a few extra packages installed (possibly dependencies) and a few are missing:
I noticed that Yarn is installed for Node.js builds, so perhaps some of these are installed only for specific languages or using some mechanism other than Chocolatey.
python2 is required for Node.js. I ran into this while investigating https://github.com/nodejs/node-gyp/issues/1568.
Some common Node.js modules need to be compiled during installation. The installed Visual Studio packages are adequate, but
python2 is missing. Linux and OSX builds do not have this issue.