Any known Issues with Merging Duplicate Customers across Subsidiaries? I would have thought NS wouldn't do this but tried it in SB and it went through with no problem.

I haven't run into any when I've done it in NS itself. If you have some external system syncing customer data via middleware, it can break that. But yeah just in NS on its own, I've never had issues. Only annoyance is having to do it one at a time.