XML can be used to represent data formats, but is missing things like types.

It's powerful enough to represent documents, which might be too much for data structures:

  <a>This is <b>valid</b> XML</a>

And let's not forget how hard it is to escape data in XML, so every value ends up as CDATA in the end.

  <config>
    <somekey type="string"><![CDATA[4byte emojii]]></somekey>
  </config>