How it's safe migrate fid and uid?
The core migrations from D6/D7 to D8 do preserve IDs by default. But, it is fragile, you have to be careful not to create any content on the D8 side before you've completed all migration. Unless you depend on public-facing addresses (e.g., /node/[nid]), it's simpler to just allow IDs to be assigned on migration. See https://www.drupal.org/node/2748609#comment-11293627 for more.