YAML is bad. It's like markdown, there are too many parsers behave differently. Unlike markdown just for reading, it is used in configurations for critical systems. JSON is much better, it IS readable and writable, people using package.json all the time without problems.
Templating YAML is even worse. Templating is an ad-hoc abstraction, and very easy to run into issues. A minimal JavaScript runtime with JSON would be much better, JSON is JavaScript Object Notation after all.
Markdown's problem is no single standard. This is not the case with YAML, so no, it is not like markdown. And you can technically write assembler also.
Templating YAML is even worse. Templating is an ad-hoc abstraction, and very easy to run into issues. A minimal JavaScript runtime with JSON would be much better, JSON is JavaScript Object Notation after all.