Proven in what sense? Several implementations are broken are incorrect. HCL is used in very large products as well. Just because it isn't the majority currently doesn't mean that it isn't a worthy choice. HCL isn't more complicated if used as an alternative to YAML or JSON, in fact, I would argue that it is simpler. It bridges the pros of YAML and JSON combined, and addresses the nested complexity of TOML. It really is IMO the best, but you of course are free to share a different opinion. However, I would encourage you to actually try it out and re-evaluate.