The fingerprint scanner position is just terrible. Why put it at the top part of near the camera. This have several disadvantages: first of all, it's harder to reach because the phone is very tall (especially in the S8+) and second, you can mistakenly put the finger on the camera housing and just put smudges and dirt on the lens, which then you need to rub off before shooting.
This is just dumb to put the fingerprint reader there. The best place for it in my opinion is at the front, but the S8+ lacks a home button and there is not much space at the bottom of the phone, so Samsung opted to put it at the back. Having said that, if you asked me where it the worst place to put the fingerprint reader, I would have told you that exact same position.
Right-handed users will find it even harder, because they will need to stretch their index finger to reach the top-right side of the device. Not that it's not possible. I believe that most people will have no problem reaching it with their index finger, but it just feels cumbersome. If you put it in the middle below the camera, there shouldn't be any issues. In fact, many phones that have a rear-mounted fingerprint reader which is placed in the middle haven't complained about it yet. Some do prefer a front-mounted one, but this weird placement have received top headlines after the announcement event in many of the leading phone review websites.
Right-handed users tried it at the event and indeed find it odd to reach that spot where the fingerprint scanner is. This is certainly a disadvantage in the design and something that you should be aware about. A deal-breaker, not at all, but an annoying inconvenience.