YAML is really so much more than JSON.

  * YAML can have several 'documents' in the same file,separated by ---
  * there are anchors and references
  * easy to read multi line texts
  * it's also a superset of JSON

I can see, how choosing YAML when you just wanted readable JSON might give you more headaches than expected.

And like someone else said, putting another template engine (or two) on top of YAML is when the real problems start.