INI-like formats are perfectly fine for config files with at most one layer of nesting/sections. TOML is a perfectly fine INI-like parser. Its definitions and support for strings, numbers, comments, sections and simple arrays are great. But its main claim to fame is extending INI to support arbitrary levels nesting of arrays and dictionaries like JSON, and IMO it does a horrible job at it.

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"

or

  [fruit]
  apple.color = "red"

It isn't always obvious which approach is more appropriate for a task, and mixing them creates a big mess.

And the more nested the format becomes, with arrays of dicts, or dicts of arrays, the harder it is to follow.