https://wccftech.com/oneplus-6t-slower-optical-in-display-fingerprint-scanner/

The in-display fingerprint technology is still relatively new and not only does it need time to mature, the costs are also relatively high right now. To dial down that price, most manufacturers opt for optical fingerprint readers and OnePlus is going to do the same with its OnePlus 6T.

Basically, there are two types of in-display fingerprint sensors that you should know about. First is the ultrasonic one which is expensive but more accurate. The one developed by Qualcomm can function accurately through displays being up to 800 microns thick, which is a huge improvement over the previous generations that could work accurately only up to 300 microns. Though it sounds good, most manufacturers shy away from it to employ some cost savings.