My lesson is that JSON and YAML are great for machine-consumption when people just need to do something once and leave it (eg config files), but far from ideal for stuff people need to edit over and over (such as script files).
I converted the input to markdown (with some small extensions) and it made a huge difference. it's less fiddly, much easier for humans to edit, and the parser can point out errors much better. for some nice examples check out https://github.com/videopuppet/examples/
In conclusion, YAML and JSON are great for machine-to-machine communication or human-to-machine for things that do not change often and aren't complex. for human-to-machine that is more frequent, we should be kinder to our users.
JSON5 seems like a decent alternative to either.
https://github.com/mbrt/gmailctl uses it for example.
I personally prefer toml
Like I said in one of the comments, final decision has not been made yet. JSON is not written in stone, plus ffcms could support more than one language, e.g. JSON and TOML.