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).