A challenging part of nearly any Drupal 7 to Drupal 8 migration is that the content keeps coming in. While you're building in Drupal 8, the live Drupal 7 site has data added, deleted and revised. Drupal's migration system is designed to handle this problem. But do you want to leave migrations running for hours on your laptop?
To know that the work in Drupal 8 is stable, you need a continuous integration process that can migrate new content, check that it migrated correctly, and test the functionality in the new site. Ideally that process runs with every change to your codebase.