Years ago I had to support a tool that used YAML as a configuration language, and a transport between different applications. Holy. Hell.
First of all, don't ever try to edit a YAML file by hand. You will introduce whitespace or other characters that will break the file, and you will not know until you run it and it breaks something.
The reason you will not know? Not all YAML parsers are the same. Some will interpret it correctly, and some will break. You'll have to get reference implementations of every "supported" YAML parser and run every config you have through them all, and diff them all, before you can trust them.
YAML may be easier to read than JSON, but its added complexity (the parser is significantly more complicated) and obtuse "features" are just not worth the effort. Not to mention, have you ever tried to maintain a very large indented YAML file by hand? Pain in the ass. Just shove everything into JSON files. The fact that it's so limiting is freeing, and everything can parse it. But don't edit it by hand.
And IMNSHO, you shouldn't use either YAML or JSON as a configuration language. They are for data structures, not configuration. If you want a configuration language, go get something designed as a configuration language.