Walt Mossberg reviewing the S8 said that in his opinion all 3 biometric types didn't work very well (fingerprint was awkward and took too long, had to open eyes real wide for iris scan, and facial recognition didn't work for him), and so I think a lot of us will end up going back to typing in a code a lot of the time.
I got an S8 to supplement my SE and have found the complete opposite to be true: Fingerprint on the back is a bit more awkward than on the front but easy to find with a case on and reads my fingerprint first time about 95% of the time. Just touching the scanner unlocks the phone and goes to the home screen.
Iris scanning is sub-second and incredibly accurate (hard press the "home button" while looking at the phone and it instantly unlocks and goes to the home screen). I haven't tried face recognition because you can't have it on at the same time as iris scanning and iris scanning has been working so well I don't see the need to try an inferior option.
Given the option of a bigger screen and the fingerprint scanner on the back or a smaller screen with it on the front, I think putting it on the back is a decent trade off. Having said that, I think if anyone can get it working under the glass and provide the best of both worlds, then Apple can and I wouldn't be at all surprised to see that's what they do