I think that being able to have separate databases for different domains is pretty good, although it does rely partly on the application layer to keep (UU)IDs consistent.

I'd be curious to know if there's a way to have databases talk to each other to just sync up primary keys for referential integrity. That could maximise the benefit of decoupled databases while still having good referential integrity. And a network disconnection would still mean existing PKs would be in place, it just wouldn't be aware of new ones. Not perfect, but not bad, perhaps.