So, why do I say that self-service BI takes 'faith'?
Whether you trust sales person 1 who prepared a report in Excel or their favourite BI tool based on the centralized data warehouse that has ingested the raw sales data, applied consistent rules to modify the data and provide it in an easy to use and understand model or sales person 2 armed with an exported sales report, Excel to modify the data and a data discovery tool to report accurately on last month's sales, you need to trust the initial data quality, interim transformation process and final aggregation and visualization.
Neither process is wrong, but there are obvious benefits to each approach and organizations should take a bi-modal approach to leverage the strengths of each.
My experience working with clients supports this. There are several types of people who work at organizations, but at a bare minimum, there are data analysts (those trained and experienced working with data) and data consumers (those who either lack this training and experience or simply don't have the time). We are not moving forward if we expect those in the second category to supernaturally 'help themselves'.