I think regardless of FaceID or Fingerprint, it doesn't really matter that much as it's pretty lax security as a whole when it comes to privacy. If your phone gets into the wrong hands when you're present, whoever has it can almost certainly compel you to unlock it by force or without your consent. With a passcode they can't compel you to tell them the code. For me that's where all this falls down in terms of personal privacy and security.
I realise it's not a concern of 99.9% of the population as they want convenience and a deterrent for thieves, and for that it's pretty great. But I think as time goes on I can see more people than just border control agents wanting to take your phone and download all the content to scan it or check for information. It makes me feel pretty uncomfortable to think that this may start to occur more often, but I can see it happening and FaceID makes it easier to access this data if they grab your phone and wave it in front of your face. Whether it's random stop and search on the streets, in offices or schools, or passing through a port. I don't know what the solution or end game will be for that.