According to the folks at SamMobile, if you are having problems where your custom watch faces are crashing when you attempt to initialize them, you might need to perform a reset on your watch. While this might not be a common issue, using custom watch faces from the likes of Facer, Watch Designer, and others could cause this problem. After you've performed the reset, you should be able to reinstall your custom watch faces and have them work like normal.
This doesn't seem to be the only issue with the update, as some apps are struggling with a bug that causes text to become misaligned. Samsung is aware of the issue and it is working on an update that should fix the problem. Unfortunately, there is no word yet on when this will land on the Gear S3.