At the end of the day, much of all payment systems is just a ledger. Payment systems have to be provably correct, reliable, and available. Many payment systems make tradeoffs between all of these. Much of existing payment systems are all batch based.

What we have built at Uber is a realtime system that is reliable, provably correct with high availability.

As I've said in another comment, for many other systems, if it fails, people can just try again later. If we fail, someone's mother, grandfather, son or daughter are stuck on the side of the road in the middle of the night somewhere. Our needs (or at least the standard that we put on ourselves) is higher than many other payment systems.