I was happy to see that salesforce is making strides to make their system more interoperable with legacy (on-site) applications and Salesforce Lighting and oData are good steps in that direction. My experience has been that there are usually concerns that are raised by either SAP BASIS personnel or from the Security team to allow for unrestricted access to the SAP system.
Master data management is one of the major factors contributing to the success or failure of business system implementations. Systems can be implemented correctly from a technical point of view but still fail in practice due to data integrity problems. This is particularly true with master data because it usually drives a great deal of functionality.