What are the biggest challenges during an AWS cloud migration?

We're planning a cloud migration project and evaluating different AWS Cloud Migration Services. For those who have completed a migration, what were the biggest challenges you faced? How did you handle application downtime, data transfer, security concerns, and cost management? I'd appreciate any real-world experiences, lessons learned, or recommendations for ensuring a smooth migration process.

The biggest issue I dealt with was hidden application dependencies that only showed up during testing. I fixed it by doing several trial migrations, documenting every dependency, and only switching production after everything worked as expected FNF.