What are the pros and cons of using Cognito-id (Cognito sub) as a primary key?

Hi,

You have basically identified the primary reason not to use the Sub attribute. If you need to migrate user pools, replicate users to another region, restore expected users, etc. the sub would be different so therefore it's typically a bad idea to use it as a key. A UUID with the Cognito ID provides more protection against this. The only downside of this is that you still have to create the unique key yourself but it avoids any issues in the event you have to recover the pool.

Thanks, Owen