I am an Ariba Upstream consultant and have recently attended and Instructor Led Training from SAP for Ariba Upstream. So far, what I have concluded is that there could be a number of implementation methodologies dependent on the customer's requirements. The main challenge is Supplier engagement, enablement and data upload.
As you know that a consultant doesn't install anything on a cloud platform, a consultant being able to understand what is possible and what is not possible primarily behaves like a bridge between the SAP Ariba and the customer.