> If you are doing a data migration and failed to account for some unexpected data, now you have people on different schema versions until you figure it out. Now, yes, if you are at sharding scale this may occur anyway, but consider that before you hit that point, a single database is easiest.

This can be accounted for and handled. Though if schema issues are enough of a scare I wonder if a documentdb style embedable database like a couch/pouchdb might make more sense.