In the XML mapping of values of Complex DataTypes, both the name of the type and the names of the members end up as names of XML elements in the XML NodeSet. Element names have to follow the rules from https://www.w3.org/TR/xml/#NT-Name, so either
1) if a Complex DataType or one of its members does not follow the rules for an XML name, the value can't be mapped to XML (something I don't like)
or
2) an official, unique, reversable mapping has to be defined from any utf-8 BrowseName to an XML Name to be used as Element name (I'm not sure if I like that - that will look ugly)
or
3) the possible mapping to XML unfortunately places restrictions on the names of Complex DataTypes and their members (and that should be documented)

There is an additional complexity here: In names of XML elements, escape sequences are not allowed, i.e. something like <Element> does not work. Therefore, the allowed characters also depend on the XML document encoding:
<?xml version='1.0' encoding='utf-8'?> would be fine for everything, but e.g. <?xml version="1.0" encoding="ascii"?> would narrow down the allowed characters of a Name.

Two possible solutions for the document encoding dependency:

1) In addition to the restrictions of XML Name, names of Complex DataType and their members shall be ASCII only, so they work with any XML document encoding
2) OPC UA NodeSet XML files shall use document encoding "utf-8" to not further restrict names of Complex DataType and their members beyond what XML Name requires