A quick follow-up,
We're still testing High Sierra.
While it does seem to work for us with many Audio Unit plug-ins (and VST).
It seems there is a some issue with Audio Unit Components and High Sierra.
We've been able to identify an issue (at least with Logic & GarageBand) where some Audio Unit components won't show up until full restart of your computer.
however, once restarted all plug-ins (including some very old resurrected did show up).
Our current suggestion as of tomorrow (September 25th, High Sierra release date) is to wait a little for things to clear up.