I am required to host most of my feature services on ArcGIS for Server on an internal DMZ that can be accessed by AGOL through the REST end point. I am also required to secure my services. Currently in AGOL you can add the REST endpoint directly to a web map, but when you view the web map you are prompted for the credential to see the layer. AGOL also allows a Feature Layer item to be created with saved credentials pointing at the REST end point, but this is an unsupported feature for Collector and is prone to issues. In recent interaction with ESRI I was told that this functionality is a bug, but not in the product plan to fix:
[BUG-000086198: An error, "Invalid URL" error is returned when attempting to download a map in Collector for ArcGIS 10.3 (iOS) when the web map contains an ArcGIS for Server feature service with credentials that are stored in an ArcGIS Online item.]
The status of this bug is listed as "Not in Current Product Plan". The suggested workarounds are to simply use the service as-is, without embedding credentials (i.e. without proxy).
What is also frustrating is that this does work part of the time, but not all the time. It does not always error, and it is not always the error above, sometimes it is illegal token [<], or just unknown error.
In my case I am limited by corporate restrictions on where I host my data and this limitation makes it very hard to continue using Collector.