The biggest issue with current in-screen fingerprint readers is they’re optical readers and not capacitive like the old TouchID or fingerprint readers located on the back of devices.
This results in a couple of issues:
First, they’re often times slower because they’re essentially taking a picture of your fingerprint and comparing it to what’s on file. In order to do so, a light needs to be shined onto the finger so that the fingerprint patterns can be "seen" by the optical sensor. In the case of the OnePlus, a green light appears where you are to place your finger, and this same greenlight is what illuminates your finger. However, if the screen is dirty, of if your finger is dirty (or wet), or the angle of your finger on the screen is off, the fingerprint reader may be slow or fail outright.
Second, the security of an optical reader isn't great. Unlike a capacitive reader, which is able to detect the subtle differences in capacitance b/w the groove and peaks of your finger (and thus can't be fooled by a pic), an optical reader can theoretically be fooled by a simple high quality pic of your finger.
I imagine, as the technology matures, these issues can be reduced, if not eliminated entirely. But, at the moment, in-screen readers are just not ready for prime time.