Why SharePoint Migration Projects Go Wrong
Most failed migrations don't fail on the move itself, they fail on what happens before and after it. Permissions get flattened into "everyone can edit." Version history disappears because a tool copied only the latest file. Users lose confidence in the new environment within a week because search doesn't return the same results it used to.
A SharePoint Online migration is a data-integrity project first and a file-transfer project second. That ordering is what separates a clean cutover from a support queue full of "where did my document go."