Just a word of caution: I had seen this post prior to investing in the new Mac Mini (2018), which also only has USB-C ports. So I purchased the adapters (nearly £100) and my old Fireface 800 is connected and delivers clean audio as expected - but sadly only for a minute or two at a time!
The dropouts are always the same: 4-5 seconds of silence, a short glitch (under a second), 4-5 seconds more silence and then we're back in the game! This is on playback via any app outputting sound - Logic, iTunes, YouTube, standalone VIs etc. I haven't tried recording yet. Software continues to run as normal with no glitches in the graphics (meters etc.) just dropouts in the audio output.
Whether this is to do with the adapters, Mojave, or the latest RME Driver is anyone's guess? Everything is up to date. Have reported to Apple but they are throwing it back to RME. Lots of speculation in the forums but nothing from RME Support in nearly a week - have tried email as well! Is there anybody working here still?? If nothing else it would just be nice to know someone is working on it, otherwise I need to consider the expense of a new audio interface.
https://www.forum.rme-audio.de/viewtopic.php?id=28006