JSON has two drawbacks: a lack of comments (although you could add "#" keys in relevant places) and no binary support (arbitrary conventions include base64) but this doesn't support binary anyway.
It isn't a friendly form of human input. My error rate is 50%+ , you have to lint on save to catch things that are invisible to the naked eye
No ability to override, extend or reference keys. This is most useful in config objects where for eg. in a dev object you want to override the username and password for a database connection but not repeat all the other parameters
No comments
Lack of comments makes JSON much better for data exchange than formats with comments.
Am I missing something with that?