TOML is not a very good format IMO. It's fine for very simple config structures, but as soon as you have any level of nesting at all it becomes an unobvious mess. Worse than YAML even.

What is this even?

[[fruits]]
name = "apple"

[fruits.physical]
color = "red"
shape = "round"

[[fruits.varieties]]
name = "red delicious"

[[fruits.varieties]]
name = "granny smith"

[[fruits]]
name = "banana"

[[fruits.varieties]]
name = "plantain"

That's an example from the docs, and I have literally no idea what structure it makes. Compare to the JSON which is far more obvious:

{
  "fruits": [
    {
      "name": "apple",
      "physical": {
        "color": "red",
        "shape": "round"
      },
      "varieties": [
        { "name": "red delicious" },
        { "name": "granny smith" }
      ]
    },
    {
      "name": "banana",
      "varieties": [
        { "name": "plantain" }
      ]
    }
  ]
}

The fact that they have to explain the structure by showing you the corresponding JSON says a lot.

JSON5 is much better IMO. Unfortunately it isn't as popular and doesn't have as much ecosystem support.

YAML is complex and has security concerns most people are not aware of.

YAML is racist to Norwegians.

If you have something like country: NO (NO = Norway), YAML will turn that into country: False. Why? Implicit casting. There are a bunch of truthy strings that'll be cast automagically.

People bitch about YAML but for me it's still the preferred one just because the others suck more.

TOML like said is fine for simple things but as soon as you get a bit more complex it's messy and unwieldy. And JSON is fine to operate on but for a config? It's a mess. It's harder to type and read for something like a config file.

Heck, I'm not even sold on the S-expressions compared to yaml yet. But then, I deal with so much with all of these formats that I simply still prefer YAML for readability and ease of use (compared to the others.)