I have to say that on the surface, this sounds horrific.

- MongoDB: Yes

- Micro Services: Yep

- Some attempt at a grand unifying model: Of course

That's pretty much a 10/10 shitshow as far as I am concerned. Mix in banking regs & regional differences for afterburner on that money furnace.

But, in reality, I doubt that one of those 65 relational databases involves the core customer/account/transaction processing facilities. These are probably (hopefully) still running on an IBM system with proper ACID & uptime guarantees. Anything a live transaction flow (especially credit/debit processing) is going to touch could never be trusted on something like MongoDB in this current reality.