Face Unlock is an feature created by OnePlus. This option allows only to unlock to your phone, so it works in the other way than front camera in iPhone X. Initially OnePlus added this option to the OnePlus 5T, and people liked it very much. After some time owners of OnePlus 5 demanded this feature in their phones, and then they received it. Now we know that's not the end of OnePlus's plans connected with Face Unlock.
Carl Pei, global director of OnePlus confirmed that his company listens to the opinion of its users which wanted Face Unlock feature also in the older phones and in the nearest future it will happen – apart from OnePlus 5 Face Unlock will be added also to OnePlus 3 and OnePlus 3T. New feature will be added to the newest open beta update.