TouchID has a disadvantage in that it doesn’t work when your fingers are greasy or wet or with gloves. FaceID would work in those scenarios. Honestly Apple should have included even TouchID as an option on the back as it has advantages which outweigh FaceID in some scenarios like when asleep or when someone has a high resolution photo of your face but likely couldn’t get it to work under glass and opted for form over function as the back fp sensor isn’t ergonomic for many.
I don't think that's an issue, it scans 3D so I'd be surprised if it could be fooled with a photo.
I have a fingerprint sensor on the back of my work phone (Android), and it works great. I wish they'd made the Apple logo the TouchID on the X. Not that it'll affect me yet, I'll be sticking with the 7S/8 unless miracles happen and the 128GB X is under £900.