ArcGIS Online should have more resources for understanding current handling capabilities for multipart polygons and then added tools for managing multipart data.
In addition to this: all editing widgets in ArcGIS Online/ArcGIS Enterprise should have support for merging features of all geometry types into multipart features and exploding multipart features to singlepart. There is a union option in the Web AppBuilder edit widget, but it only works with polygons. I have a use case where I need users to create multipart polylines - this is not possible currently.
