I've often reached out to TOML for truly minimal configuration files, so I'd say TOML makes sense in a lot of scenarios. However, my point in the article is that simple stuff eventually stops being simple, so you need to be careful.

As an example, a friend of mine introduced TOML to a reasonably big open source project a while ago. Recently, he mentioned there were some unexpected downsides to TOML. I've asked him to chime in here, because I think he's more qualified to reply (instead of a best-effort attempt from my side to faithfully reproduce what he told me).

I think for a small configuration TOML might be fine. Where it breaks down in my opinion is in larger configuration files. It just becomes pretty unreadable.

Think about a Github Action being written in TOML ... would probably not look great!

I don't understand that either. Considering that it is both well defined and hierarchical, with none of the problems of YAML or JSON (hey, NO is a country, not a truth value), TOML seems like the perfect compromise to me.