We use Mulesoft where I work, and XML namespaces are a constant issue. We never managed to define an API spec in such a way that the RAML compiler and the APIKit validator would both accept the same payload. In the end we just had to turn off validations in APIkit.
Namespaces were fun! But mostly used for over engineering formats and interacted with by idiots who do not give a toss. Shout out to every service that would break as soon as elementtree got involved. And my idiot colleagues who work on EDI.