It’s inconsistent and annoying. Expressive, yes. Gets it’s job done, yes. Absolute nightmare of a spec, YES.
The fact that JSON is a subset of YAML should tell you everything about how bloated the spec is. And of course there’s the “no” funny things.
Personally, my favourite way to write configs was using lua (because it was already part of the project so why not), but JSON does fine.
Personally, I prefer duplicate keys to be eaten by the parser but I can see how it’d be beneficial to prevent them.