One pattern I’ve noticed is people seeking a language that’s better than {JSON,XML,INI,etc} at wrangling their slightly complex configuration files, noticing the additional features and type support offered by YAML, and assuming it will be a good solution.

Then, as their configs grow ever larger and more complex, they discover that expressing them in YAML requires large sections of deep nesting, long item sequences, and line wrapping. The syntax style that they saw working well in other places (e.g. certain programming languages) breaks down quickly at that level of complexity, making it difficult for humans to correctly write and follow, and leading to frequent errors.

YAML doesn’t suck for small stuff, IMHO. (But it is more complex than necessary for small stuff.)

For things likely to grow to medium-large size or complexity, I would recommend either breaking up the data into separate files, or looking for a different config/serialization language.