Comment by pseudohadamard

YAML also expanded to add arbitrary scripting via a pile of bolt-on capabilities so that it's now a serialisation language that's Turing-complete, or that includes Turing-complete capabilities within it, everything from:

  command:
    - /bin/sh    
    - -c
    - rm -rf $HOME

to:

  state: >
    {% set foo = states('...') %}
    {% set bar = states('...') %}
    {% if foo == FOO and bar == BAZ %} 
    ...

This makes it damn annoying to work with because everyone's way of doing it is different and since it's not a first-class element you have to rethink everything you want to do into strange patterns to work with how YAML does things.