Face ID is much better than Touch ID for my use cases, but I still hope Apple brings Touch ID under the display for future devices purely to have an alternative or additional authentication method.
Anyways, regarding Face ID if you go to unlock the device in terrible lighting conditions (such as really bright outside) and you are looking at the device, it would know that you are looking at it but it does not have a clear enough image to get an accurate facial recognition scan.
What Apple should allow is extra software, maybe via machine learning to know if a shot is overexposed, that it should scan for your face more frequently.
Currently if you are in this scenario, you have to either tilt your device down and back up again or swipe to authenticate which could possibly fail again.
If Apple made it so in these scenarios it checks for your face every 2-3 seconds, the device would authenticate a lot quicker and provide less of a reason to use a PIN to unlock.
For a point of reference, I probably need to put my PIN in once a month, unless I reboot my device, where as with Touch ID on my iPhone 5s, I had to put in a PIN up to 12 times a day.