The popular Apple features you won't be getting with the iPhone Duo

–Engadget 

The biggest loss you'll feel when switching from a traditional iPhone to the Duo is the latter's lack of a FaceID array for facial recognition. If you've grown accustomed to FaceID, which pretty much always works as long as your face is somewhere in front of the screen, losing that option could be a major blow. When unlocking your phone, making purchases and logging into apps requires fiddling with a finicky fingerprint sensor every time, it can become aggravating. On the other hand, some users still miss the older iPhones with TouchID. Maybe your face doesn't play nice with FaceID for whatever reason.