These under-display sensors aren't flawless, as there tends to be a delay compared to a reader that's in direct contact with your digits. Synaptics isn't bothered by that, though -- it claims that Clear ID is "twice as fast" as 3D face recognition (i.e. Face ID on the iPhone X) and that it's more flexible, since you don't need to be within visual range of your phone.

As it is, the technology might be vital if it's widely adopted. Now that tall-screened phones are practically de rigeur, phone makers have usually had little choice but to move the reader (typically to the back) or else use another biometric sign-in method. Clear ID theoretically lets phone brands avoid that choice. They can put the reader where it's most convenient without giving up that all-important eye-catching display.