AFAIK, NS8 will ONLY accept itself as AD for the moment, a Windows AD or anything else (like an exotic Synology AD) won’t work! This is at least valid for Beta1. Later releases may allow more options…
See here, the last post…
Note: I use Synology a lot for my clients, and it works well. Biut then, I’d NEVER use a Synology as AD! I use it as Storage, and a very few Apps (on private Synology NAS).
Some Synology Apps work well, and are well designed and are usable, others, like for example the DNS Server is hardly usable. A random Hostname for the DNS Server which you can’t set, even as administrator? An extremly kludgy interface, not friendly if you need to maintain 10-20 DNS entries…
The very popular Media Apps: Photos, Audio Station and Video work fairly OK, albeit a bit slow. Using eg Jellyfin on the same NAS shows a much more responsive Interface and works well!
My 2 cents
Andy