The other approved XML syntax for invoices in the EU is UN/CEFACT CII (United Nations Centre for Trade Facilitation and Electronic Business Cross Industry Invoice).
Unlike UBL, the scope of CII is restricted to invoices; other document types are not supported. But that does not mean that the format is in any way simpler than UBL. In fact, the CII documentation is certainly not for the faint of heart.
CII can carry an overwhelming amount of information going far beyond the requirements of the European Standard EN16931. Currently, it is not possible to produce these additional elements with E-Invoice-EU.
The documentation for Factur-X and ZUGFeRD describes the syntax and semantics of CII according to these standards but reading and using the documentation is a challenge of its own. Since E-Invoice-EU does not require a deeper understanding of CII, this is left as an exercise to the reader.