I agree with Lars when he says that allowing lists of formats would overly complicate the UI.

In line with the "don't break UI functions" philosophy I suggest we interpret --exports=xml as "export to XML where possible and do nothing otherwise".  It is always possible for the user to export any remaining output explicitly.  As suggested by Michele we might emit a warning when skipping a non-XML output.

For the future we might consider supporting an option such as --exports=default which means "export all outputs in their default format".

Yet another possible future approach might be to allow the user to specify a configuration file which specifies their preferred formats for different output types (e.g. loss maps in GeoJSON, loss tables in CSV, everything else in XML).