Genuinely curious - What major flaws does TOML have? I've used it before and it seems like a simple no-nonsense config language. Plenty of blog articles about the flaws behind YAML, I don't really see complaints about TOML!
With JSON, YAML, XML and many other formats, the syntax for nesting has a visual appearance that matches the logical nesting. TOML does not. You have to maintain a mental model of the data structure, and slot the flat syntax into that structure.
Furthermore, there are multiple ways to express the same thing like
[fruit.apple]
color = "red"
[fruit]
apple.color = "red"
And the more nested the format becomes, with arrays of dicts, or dicts of arrays, the harder it is to follow.
While I have some minor annoyances with TOML, I counterintuitively consider it a strength of the format that nesting quickly becomes untenable, because it produces pressure on the designers of config file schemas to keep nesting to a minimum.
Maybe some projects have a legitimate need for something more complex, but IMO config files are at their best when they're just key-value pairs organized into sections.
The various features it has for nesting and arrays make it convenient to write, but can make it harder to read. There is no canonical serialization of a TOML document, as far as I can tell, you could do it any number of ways.
So while TOML has its use for small config files you edit by hand, it doesn't really make sense for interchange, and it doesn't see much use outside of Rust afaik.
Sampling bias, there are no complaints about it because no-one uses it (jk).
It's subjective of course but despite the name TOML never really seemed that 'Obvious' to me, in particular the spec for tables. I also think the leniency in the syntax isn't necessarily a good feature and serves to make it less 'Minimal' than its name suggests.