NewUserMessage tried to edit my user talk page, but not properly through Flow. So I got a notification, but no actual message:
This particularly affects gomwiki/Konkani because Flow is enabled everywhere there, so user talk is Flow even without a Beta Feature being turned on (a user turning on the Beta Feature would turn it on after NewUserMessage was done).
It could affect other wikis, though (e.g. it would affect MediaWiki.org if NewUserMessage was enabled).