Work takes time but finally getting around to answering with tomorrow being a holiday.
I definitely want to have an open site. (Some) Users don’t always want that and like to be in their own sandboxes. Given time that hopefully changes so I’m not imposing restrictions.
If these groups of people have literally nothing in common, then no, I actually wouldn’t shove them all in the same house…
Maybe I’m trying to be too effective. Thing is, when you have many minimal forums the cost issue alone makes it worthwhile to bring them all under one umbrella. You can then manage with one set of instructions, one site to maintain… the benefits are numerous. And I’m certainly not doing this because I’m lazy, just as an example of the new problems I ended up with… users are using both Finnish and English so I have to duplicate everything that is important enough in several languages (not adding Swedish and Arabic yet 
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… make no sense to me, the public areas are public, that is not a “leak” that is normal.
Yet, users complain, almost not at all though (the scare is of course they just silently go away considering this site to be spamming them). This is why I could see a need for two different levels of notifications. One that adds any new posts as a notification. One that only adds notifications from areas that you have visited. If you would like to force people to acknowledge some new category you could always do some sort of “@ all” in it’s header to wake people up.
Let me rephrase:
A previous Discourse site has so much traffic you got notifications all the time as new categories and everything got added on daily or at least weekly basis. I would suspect this is common when a new site is set up and hundreds or thousands of users appear in a short time period. The site naturally lives. But, it’s also very discomforting to new users when they get swamped by notifications. In the beginning you probably only want notifications from the stuff you are really interested in. Later on you might have time to explore. Personally I turned off notification mails altogether as I started to drown in the “new stuff”. And I consider that bad.
Finally, one of the reasons I’m having problems is that originally I expected new groups to come in and join certain closed ares in batches. So I could set a default primary group and they would immediately see their own stuff. Setting a default group newer worked, I had to tell users to wait until I see they have arrived and then manually add them to their correct group. Users also arrived though my SSO solution which means they can select their alias during the authentication procedure. This also means they do not actually appear on the forum until they have authenticated themselves, so I could not attach them to a group in advance.
If there are known problems with the primary group getting assigned upon first login I can investigate more. Upgrading to the newest version right now anyway in the background (thus some time to write).
Got a bit lengthy, but should give some understanding that Discourse is good enough for people to try to bend it to what they actually need 
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). I suspect most users, even when instructed how to, are not capable of doing that. Since I actually think the digest is useful, when not having too much unnecessary stuff, I’ve set the User preferences default to 3 days instead of 1 day but that’s about the best I’ve been able to do (more than explain how to remove the digest altogether).