Author here. I understand your concerns, and in fact one of the last things I added is unit tests: https://github.com/mbrt/gmailctl/blob/master/README.md#tests

Getting things right is not easy, especially in complex languages like jsonnet. In my own experience though, every time you limit the expressivity of config languages you end up inventing even more complex workarounds in the end (e.g. external templating and things like that).