Part of the history of the XPS backend was to have more than one
non-bitmap backend so that we could figure out what the SkDocument
interface might look like, as well as helping influence the design of
the SkPDF back-end, which was originally written quite differently.
Also, it was written before Pdfium and the XPS backend was one
possible way to improve printing in Chrome which Chrome could have
some influence on. Since then Pdfium happened and Chrome can do pretty
well with just PDFs. Since printing generally doesn't get a high
priority and isn't a particularly popular thing to work on it just
hasn't had the time put into it. That being said, the XPS backend
still mostly works for common things (though isn't well tested), and
if it were a priority many missing bits could be implemented
(somethings like font subsetting could really be updated as well). So
the answer to your question is that it's probably not a great idea to
use it in production, but it probably could be made usable with a bit
of work.