Deploying Survey123 on premises is a great possibility, allowing organizations to keep all ESRI resources & traffic within the organization (for instance, this is a legal requirement for many governmental bodies outside the US).
Currently (oct-2019), the beta available on the ESRI Early Adopter Community (ref. here) lacks one of the major functionnalities: the ability to create feature reports, i.e. complete a MS word document model with attributes, attachments and location preview form the feature service.
With a on-premises deployment of Survey123, the only possibility is then to run simple merges between an MS Excel data extract and the word document. Attachments are not available, and location cannot be rendered as easily/directly: the report is much poorer than what can be done with Survey123.arcgis.com.
Please add feature reports to a not-so-distant future release of the Survey123 Enterprise (On-Premises) build!