[JonChesterfield]

Msgpack is broadly fine (I shipped a parser for it a while ago, relatively few ambiguities in the spec). Json is kind of ok if you don't do numbers but schemas for it are a pain and the tooling gives me hacked together vibes.

I like being able to read and edit the data files easily in a text editor (bias against the binary formats) and for there to be a decent chance tools written by other people will interact predictably with the format (so it can't be bespoke).

I'd say the main feature is that an XML document with a schema tells you a lot about the various shapes of the file that you might need to worry about. It's essentially an extensible type system for tree shaped data.

XML has an annoying collection of spurious limitations but that's what you get with lowest common denominator / popular-cos-old systems.