In Web Services it is common practice to use variable user-defined attributes. It is not possible to use this in Uniface. So a call out to a Web Service that uses variable user-defined attributes is not possible because it is not possible to fill the necessary attributes.
It should be possible to read and write variable user-defined attributes with the xmlsave/xmlload proc.
Supporting only tags and fixed value attributes is only part of the XML story and rules out the call-out to a number of Web Services. Of course it is possible to use some sort of an intermediate WS wrapper in another programming langauge that can handle variable attributes, but that complicates business and maintenance.