ArcGIS Hub needs to support any formats that ArcGIS Online supports. For example it currently does not support zip files. The zip files can be uploaded to ArcGIS Online through "Add Item" and then shared to the Public and to your Open Data group but ArcGIS Hub will not recognize it. The workaround is to attach it as a metadata item to another feature service. It would be much better to access it directly. The nature of "Open Data" is that it should support a wide variety of formats (i.e more than the 7 currently supported formats).