Not the person you’re replying to, but I would expect that a near universal answer to this across all kinds of projects (not just software) is effective collaboration and communication between stakeholders and teams.

Despite no shortage of technical talent on large projects they can still often fail, and it’s because building a technically impressive thing doesn’t matter if it doesn’t do what business needs.

So it’s about making sure you’re building the “right” thing that delivers on business’s actual needs, and the only way to find out what those are is through constant and ongoing good communication between technical and business people.

Specifically for the financial sector and especially banks and government tax departments, they’re on a clock.

As time moves on, there are less COBOL engineers. Hell, sometimes their systems have been written in a bespoke language. There is less and less understanding of why something is set up the way it is due to lack of documentation. Updates / changes to the code sometimes have to wait for 2-3 years because the system isn’t flexible enough (literally, not as in “this change will take 2-3 dev years”). Even code that old contains bugs, but due to the age of the code they’re inscrutable.

However, whichever new system gets tooled up has to be 99.999% flawless, or it could cause serious damage to the bank and even its regional market.

When there is that kind of pressure, dev teams are no longer considered a cost sink, money flows, and the world is possible.

A large project where the end goal is replicating (and possibly correcting) existing data outputs is much more likely to succeed than one that is integrating new data sources or building new data models. For the latter type of project, it's very common to find that the team is disconnected from the business users and original motivation for the project, with poorly defined success criteria.

There was a clear, large cost savings and risk improvements with the project. The project was actually easy on the requirements front. They put all new non critical features on hold for 2 years and there was no question on the requirements: The new system's data must match the old system's data except for any bug fixes or agreed upon changes.