Bishops’ Conferences are relatively new upon the scene so no power was ‘returned’ to them. Vatican II gave them some restricted authority. Theoretically a bishop has ultimate authority under the Pope alone in his diocese.
The danger of Bishops’ Conferences is the creation  of National Churches as in Orthodoxy and Protestantism. We can see this in the diverse responses of a number of  Conferences with regard to Amoris Lætitia and the German Conference’s threats to go it alone.