The Sentinel Explorer App is an outstanding web app tool for monitoring natural resources, almost in real time. I personally use it to keep tabs on logging operations in the watershed I am interested in. While it's a pretty solid application "as is", I can't help but think it could use a few enhancements that won't happen unless it becomes an officially supported application.
The application has the ability to overlay content from ArcGIS Online but that content inherits the default symbology of the service.This becomes a problem with polygon overlays which obscure the imagery with no way to alter it. A second issue is that at some zoom levels (or some acquisition dates) the Sentinel imagery is too dark. It would be great if there was an ability to adjust the imagery's gamma or brightness/contrast. Once again, this won't happen without ESRI's official support.