Samsung's ultrasonic in-display fingerprint sensor itself is pretty good, and it works when your thumb is wet or dirty unlike the OnePlus 6T's in-display fingerprint sensor. I only say "pretty good" because it simply refuses to register my fingerprint while I'm in bed, for some reason.
There is one thing that significantly slowed down the unlocking process, to the point that I simply used the faster facial recognition most of the time: I had trouble with consistently hitting the scanning area with my thumb. It takes forever for the fingerprint symbol to show up after picking up the phone, even with the always-on display feature turned on. Muscle memory works most of the time, but not so much in the dark, at least with the Galaxy S10's in-display sensor.
So far, the OnePlus 6T still has the better in-display fingerprint experience. The fingerprint symbol appears immediately when I pick up the phone, and I immediately know where to place my thumb.
Thankfully, that's something that can easily be fixed with a software update, and I hope adding fingerprint sensor guides gets on Samsung's list of things to improve.
One thing about in-display sensors in general: Having to look at the phone — even with muscle memory — to make sure I'm placing my thumb in the right place makes the unlocking process feel slower than using the good old-fashioned capacitive fingerprint sensors. With the Pixel 3 and other phones that use capacitive sensors, I can pick them up and feel exactly where I need to put my finger without looking, and they feel faster to unlock as a result. There's just no physical guide or feedback for where you should put your thumb on phones with invisible sensors.
In-display fingerprint sensors may not be as picky with the dryness or cleanliness of your fingers, and it's really nice to have the sensor on the front of the phone again, but it does feel like one step forward and one step back.