Here's background. The project required migrating over 7 years' worth of transactional data (sales order, returns, and so on) over 7 years. The CFO was adamant about migrating the data and wanted it to report, provide year-on-year analysis, and undertake returns processing (they sometimes allowed returns up to 7 years).
The following were the challenges:
The data quality in the legacy was bad (it always is due to bugs in the past and so on. This is the reason the company was moving to the new system).
Another challenge was selling the solution. Even though the CFO was pushing for a high volume of data migration, nobody was pushing back with the facts of the legacy data, the impact of doing such a migration, and the ways to meet his requirements. Just refusing to migrate so much data is not enough; you need to convince your leaders. I would rather spend more time on this part to get it right than spending the humongous amount of effort...