SaltStack also has the JINJA2 template embedding which can make it very difficult to understand which parts of the lifecycle run through templating. I'm still not certain I understand how it works.

The most recent offenders for bastardizing YAML I have seen are the different CI services:

* Circle CI using moustache-like templating and interpolation with things like {{ .Branch }} available in certain steps [1]

* GitLab CI adding an "include" type directive to declare YAML dependencies [2]

I've also experienced this professionally. At my last company, somebody decided to add a feature to enable interpolation in some parts of the YAML deployment data. It ended up being used by a handful of people who were confused why interpolation worked in some places and not others. The weird trend of "extending YAML" seems to be going against any sort of benefits you might have by trying to use it.

[1]: https://circleci.com/docs/2.0/configuration-reference/#save_...

[2]: https://docs.gitlab.com/ee/ci/yaml/#include