We're trying to assist a client in writing a Python script that reads and writes from an AGOL feature layer to a SQL database and vice versa without having to use ArcGIS for Server.
Syncing data between AGOL/Portal feature layers and non-Esri data format is possible, it just takes a lot more work than if you are syncing using an Esri format. One workflow is identified in the updating_features_in_a_feature_layer | ArcGIS for Developers
