Each individual problem listed isn't that hard on its own. Some are even trivial. However there are so so many constraints one has to keep track of so system co

Each individual problem listed isn't that hard on its own. Some are even trivial. However there are so so many constraints one has to keep track of so system complexity quickly snowballs into something unmanageable. Each cog is simple, but the whole clock is insanely complex. And by the way, the clock can't stop and has to be running 24/7/365 while you keep changing it. Your code will quickly turn into a tangled up mess of stinky petrified spaghetti, so you will end up re-building your entire system from scratch every few years. And with each rebuild you will keep having the same trivial problems you already solved again and again and again. Billing problems in general are a subset of problems one has in banking because banking system basically = billing + interest calculation + payments + accounting + multiple equally complex things and it has to work in real-time in a super-regulated environment while working in perfect sync with other systems you don't control. If you do everything just right and manage to keep the can rolling down the street for sometime, there will be a moment when regulation changes or your business changes and your architects suddenly resign and you will end up with a mess which requires more engineering dollars than off-the-shelf system would. Don't build your business on such foundation. It is XXI century already, no need to re-invent stuff which is readily available for peanuts. Yet, every new neobank comes up with their own shiny corebanking ledger because they can't be bothered to look into somebody else's petrified spaghetti.