Got hands on experience with this. Wasn’t my design choice but I inherited an app with a database where one of the keys was tied to a completely separate database. I mean at the time it probably made sense but the most unlikely of scenarios actually happened: that other database, the one I had zero control over, was migrated to a new platform. All of those keys were synthetic so of course they were like, “Meh, why we gotta keep the old keys?” So post-migration my app becomes basically useless and I spent 6 hours writing migration code, some of it on off hours, to fix my data.
So it’s questionable whether a foreign key of a completely different system is a natural key, but at the very least never use a key YOU don’t control.
Even in this scenario it’s feasible for standards to change. ISBN-15 becomes a thing and suddenly you have books that never get an ISBN-13 so your primary key constraints cause an error for trying to insert a null. Granted, you can see a lot of these changes coming but again, they come on a schedule you don’t control.