The glass backs on the new iPhones look way too reflective for my tastes (at least from the photos I've seen so far).
I like the matte black that they currently have on 7/7+.
I'm also concerned about durability of the glass back but we'll have to wait till the phones are out in Real World (tm) to see how it fares.
I'm also mildly skeptical about the smoothness and reliability of Face ID vs. the current iteration of Touch ID but that will also have to wait till the hardware and software are out in the Real World (tm) used everyday by people in normal situations.
@grainloom yes, I'm aware of the downsides.
And as you mentioned, it is a compromise of convenience vs. absolute security.
It's a tradeoff because IIRC research showed that people opted to use no passcode at all before fingerprint sensors were available on the iPhone because it was inconvenient.