Also, as munnarg says, it will depend on your fingers. Take the V20, for instance. The first device I tried with a fingerprint reader on the back that everyone hailed as being nicely placed and that your fingers just 'naturally' rested there. Well, I must be unnatural because my finger never rested there unless I strained it. It ended up actually, physically hurting after a week, and it didn't last two weeks before I traded it back in for an S7 Edge.
On this one I still don't like FPS on the back (although the case helps to find it), BUT if you are used to holding your phone with your left hand, this will not be an easy task and I can almost guarantee that your finger will hurt after a while.