This is one of the few HN discussions where I feel a little bit qualified to give an opinion :)
Two years ago I started a small data quality checker software where users could define their alerts, frequencies,.. all in config files instead of modifying code.
I initially chose JSON as config format, but then realised comments are necessary to guide users in defining alerts. I moved to YAML, but after some "indentation incidents" started using HOCON conf [0] and never looked back. I don't see any reason for choosing YAML over one of JSON or HOCON, except being forced to because of some dependency. Features such as inheritance and text block support which were essential for me are nicely supported in HOCON.