I have come across two school of thoughts on this issue
1. Why have an overhead of updating the Record of Origin or Source of Truth when the BPD is the actual Process Container and holds the latest and greatest version of the data, also there is an overhead involved since after every Data Change Activity e.g. Human Tasks we need to update the application database, also we can use REST APIS to work with the data when we use BPM Processes as the source of truth. Also if the system source of truth is only updated via BPM and there is no need to update it from external applications/systems this approach makes sense.
2. Transaction management is not inherent to the BPM processes and the the BPM processes are not designed for efficient and safe reading and writing of the data simultaneously when processes are operational, also concurrent read and writes have to be kept in mind when we are using BPM as a source of truth. Actually by design IBM BPM is not meant to be used as a source of truth.