Uh, security at multiple levels, dispute resolution, dealing with inaccurate floating point math, CAP theorem limitations, throughput… And those are just the challenges I can come up not having worked in the financial clearing domain.
No. You cannot just build one of these at a code jam, you cannot launch a startup to build one of these in a few months. It’s a system with one of the highest fidelity requirements outside of medical equipment. Even space technology is allowed to fail for being off by a little bit. Financial systems at scale are hella difficult.