My reasoned objection to XML, having used it for many years, is that it is unconscionably expensive to parse. At scale, XML is extremely computationally expensive in almost every dimension. It is not a good way to represent almost any non-trivial data structure, but that is frequently its sole use case. XML is a great way to expand your carbon footprint in the data center.

Today, there are very, very few cases where I would recommend XML despite it being widely specified for standards.