In my experience this is only true if the UUID is generated by the database system in a controlled manner. It is not uncommon to have a unique identifier be used as a key in a database that comes from an external source. Consider an environment where you have a database that stores data for a monitoring system. So you decide to use the machine ID (a UUID) as a key. In your design you fail to take into account sys admins that do not clone a VM the correct way, so you wind up with two different machines with the same supposedly unique identifier. Sure, if you have a unique constraint on the key, you will find out about it pretty quickly, but it is just one example.