Schema needs to be downloaded, converted, adjusted
This creates a branch of the original schemas which needs to be maintained and kept in sync with the source
Adding new language to our schema requires adjustments in the parent (XHTML) schema
Only someone how knows very well a complex language like W3C XML Schema is able to create and maintain such compound schemas
We can't just reuse existing schemas for involved grammars (conversion, adjustments)
Our adjusted schemas are not easy to reuse for a different compound language. For example, in case XHTML should be embedded in SVG (other way around), we need to change a lot in our schemas and again create and maintain a branch.
Difficult to understand
Constraints for the Compound language are hard-coded within constraints for the individual languages.
This is difficult to read and understand