There are two types of underscreen sensors. The ultrasonic ones work great, even better than the back sensors I've used. Your finger taps and it's unlocked, simple as that.
The second type are the optical sensors, and these are the bad ones. You tap it and have to hold for the scan to complete, but it might fail because the screen is slightly smudged, or the light didnt hit your finger on the right angle, the room might be too dark/bright...