Because migration is a scenario that may go horribly wrong, you should be prepared to tackle it in small pieces. So often (especially with relational databases,

Because migration is a scenario that may go horribly wrong, you should be prepared to tackle it in small pieces. So often (especially with relational databases, I don't have much experience with the NoSQL realm) everything is in one logical and physical data store, even for non-related features. That makes any migration harder than it needs to be.