Nothing to worry about
This is actually probably quite difficult to do for your average thief. Id be comfortable with face ID if I knew this was the length someone had to go to get past the lock screen on my phone. And surely this is harder than obtaining someones fingerprints and replicating them.
Im quite impressed by Face-ID to be honest. I was worried it was going to be like its Android counterparts that can easily be fooled by a photo and often require you to line your phone up exactly to your eyes. Apple appear to be pioneers of this tech. Which is odd because usually they wait for it to mature on the competition before implementing it on their own devices.
Still wouldnt get an iPhone X though, first gen Apple products ought to be avoided in my experience. The iPad 1 and the Apple watch are good examples as to why.