It seems that we are moving the problem into the data layer. This is fine but we are back to design problems/choices associated with use of databases. Further, part-process will affect atomicity and which “component” should handle atomicity is unclear.
As for data flowing thru the system, this is easy if data does not come in pieces, but data often comes in pieces.
By “unidirectional”, do you mean the data is monotonic, i.e., it keeps on increasing or keeps on decreasing but does not fluctuate? However, most data is unlike this.
While one could structure a FaaS based system to address these issues, I suspect it would no simpler than a non-FaaS based system.
I am not saying FaaS is wrong. I am merely trying to understand what niche does FaaS address — what problems does it solve and what problems does it create.