For most of his career, my brother worked in a large public service organisation. Although this organisation was present nationally, it was divided up into regional units that didn't talk to each other. A 'customer' could freely operate in multiple jurisdictions with neither knowing what was happening. This wasn't satisfactory, so the idea was born to build a national computer system, with one national database. A revolutionary idea in its day!
As an experienced employee my brother was co-opted onto the team tasked with defining the new computer system, and pretty soon he was given the leading role - to spec the system and build the first prototype. And he got it to work. Different organisations could add, amend and read data, which was available to all.
But it was just a prototype and the full system needed to be built and rolled out nationally.
So the big bosses thanked my brother for his work and called in the consultants to spec and build the final system. He was asked to act as an advisor. The consultants took the prototype and trashed it; said it would never have worked on a national scale with the volume of data being added and used. They were dismissive of his efforts and threw away his spec, so my brother sat back and kept his own counsel, while the professionals went to work.
After two years of heavy consultants' fees the new system was unveiled. It didn't work. An assumption that was made early in the project turned out to be wrong and the new system spewed out incorrect information. My brother duly explained to the consultants why this happened and what they should have done to avoid it. Eventually, after more money and more time and a bit of collaboration, the system worked and was launched. It was late and well over budget. The big boss was shown the door.
The moral? Trust the people in your organisation. Don't assume that spending big bucks with external consultants is going to get you a better result. Listen to the people who are going to use what you're building.