JSON requires constant quoting, can't support multiline strings, has no comments, has no/little typing (e.g., no datetime type). It's not good if a human needs to encode data.

For configs, I think TOML beats YAML hands down; I think YAML's spot is at encoding data structures that humans need to read/write.

I do agree that YAML, the spec, is fairly complicated. But YAML, as used in most projects, by most people, is not, and can be picked up fairly quickly. It is easier to visually read as it removes much of the clutter that would exist in the comparable JSON. It isn't typically necessary to know the entirety of the YAML spec to be useful with YAML, and most of the parts you won't know will get introduced by an obvious-looking sigil, which can be used to figure out what you're dealing with.

When I've actually sat with folks struggling with YAML, it's almost always in configuration tools, and it's also always around the templating bits. Ansible, in particular, has a bizarre templating: it happens after YAML parsing, which is not the mental model most people use when approaching it. I've also found that most of the people I've spoken to intertwine YAML and Ansible's templating functions, thinking they're one in the same.

I do not think Ansible makes good use of YAML: I would rather write task files in an actual programming language, since they are — at their core — a program. (The tasks do have some metadata attached to them, but the core task itself is a program. A function in some real language can get metadata attached to it in a number of ways, and that would be a better solution.)