TOML and YAML both have the problem that if you receive an incomplete document, there's a decent chance you can't tell. JSON doesn't have that because of the closing curly.
Every time I have reached for TOML I have ended up using JSON. The first reason is that Python standard library can read but not write TOML, which is generally useless for me. The second reason is TOML does not add any benefit over JSON. It’s not that much easier to read and IMO JSON is easier to write by hand because the syntax rules are completely obvious.
The very first moment that I had to use JSON as a configuration format, and I was desperate to find a way to make a long string into a JSON field. JSON is great for many things, but it's not good at all for a configuration format where you need users to make it pretty, and need features like comments or multi-line strings (because you don't want to fix a merge conflict in a 400 character-wide line).
