I agree that YAML is painful and it really seems like it's had a lot of feature creep.

JSON is painful in its own way too, though. There's a lot of syntax noise from things like braces, quotation marks, etc, so it's easy to make a mistake. Regular JSON doesn't allow trailing commas.

YAML tried to solve some of that, and did succeed in some ways, but introduced its own issues.

TOML seems great to me, but maybe it has its own issues. TOML actually has defined data formats for things like dates (both offset and local) and times, which is missing from both JSON and YAML so every app ends up doing it its own way.