A friend of mine introduced TOML to a reasonably big open source project and he mentioned there were some unexpected downsides to it. I've asked him to chime in here, because I think he's more qualified to reply (note that I pointed him to a sibling comment that's also asking about TOML, here https://news.ycombinator.com/item?id=45295715).

As for Apple Pkl, I think we share the goal of robustness and "toolability", but pkl seems way more complex. I personally think it's more useful to keep things simple, to avoid advancing in the configuration complexity clock (for context, see https://mikehadlow.blogspot.com/2012/05/configuration-comple...).