I’ve become ambivalent about relaxng, I used it a bunch because I like(d) the model, and the “Compact” syntax is really quite readable, and it’s a lot simpler than XML schemas.

However the error messages, at least when doing rng validation via libxml2, are absolutely useless, so when you have a schema error finding out why tends to be quite difficult. I also recall that trying to allow foreign schema content inside your document without validating it, but still validating your own schema, is a bit of a hassle.