If it’s done right then I would expect Face ID to be as fast, fewer false positives, fewer fails. I don’t know if Apple has accomplished these goals but if they did then I'd consider their implementation innovative and would not assume it was done due merely to an inability to integrate Touch ID into the display.

It's impossible to be as fast as "already unlocked by the time you see the screen." It could perhaps be "only imperceptibly slower," but it can't be as fast.

This review suggests that it's not "only imperceptibly slower," with its talk of the De Niro look.

For me Touch ID has been only slightly better than using a code for unlocking the phone. It can be as fast as Touch ID in the sense that by the time you can physically get the phone in position to do something it’s unlocked. You can’t unlock it when it’s in your pocket like you can with Touch ID but with Touch ID you still have to look at the phone to verify whether or not it has been unlocked. And most things you do with the phone require looking at it.

I don't understand why they did not put the touch sensor on the back.

Because that's a bad place for the sensor. If I have to pick up my phone and touch the sensor on the back, I might as well use FaceId.

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.

I think they regard that as surrender to Android.