Well, unless you're taking your phone out of your pocket.
Seriously, the "back of phone" placement for the fingerprint sensor has downsides when it comes to "phone on the table," but it's awesome for the pocket. I just got a Pixel 2 XL, which has a more reliable fingerprint reader than any other one I've used (comparison points: Nexus 5x, iPhone 6+, 2017 Macbook Pro touchbar), and it's amazing. My phone is utterly reliably unlocked by the time I clear it from my pocket.
And a back fingerprint reader + the FaceID system would be great in combination. The FaceID system could compensate for the back-of-phone fingerprint reader's weakness when it's on your desk (or when wearing gloves/hands wet).
My 7+ also unlocks fast and reliably every time. I sometimes purposely use a different finger to make sure it still works.
I've tried the back of phone ones and they just do not work for me. For me, back of phone is a downgrade. I guess having both FaceId and back of phone would not impact me, but only going back of phone would have me sticking with the older phones as long as possible.