Not everyone seems to be happy with the fingerprint scanner on the Samsung Galaxy S8. According to some reviews, the scanner's strange placement and inconsistent results make it slower to verify users' identities.
http://www.notebookcheck.net/What-s-wrong-with-the-Samsung-Galaxy-S8-s-fingerprint-scanner.213832.0.html
Not an issue for me. As long as it's connected to my zenwatch 2 smartlock bypasses the problem.