Good observation. Yes, it is possible to turn off various types of notifications via Notifications app in your Studio. There is a long list. Here at UNA.IO we have all of them turned on, since the community is still relatively small and we have the opportunity to observe, test and work out the list of most important notifications, which will then be activated by default in future UNA package updates.
As for the general need for such variety of notifications... it depends on the size of the community, number of installed apps and patterns of behaviour. For example, on Facebook you actually do see all of those "friend did this..." notifications even if they're not related to you - you see them in the sidebar Ticker. Now, we don't plan to create a similar ticker, so we just need to work out a good combination of default notification settings.
A thorough update of entire notifications system, settings, push and email notifications is planned for the Notifications-themed sprint. So far it's voted to be the first sprint after the v9 release - https://una.io/page/view-poll?id=8.
You say "a good combination of default notification settings" but I do not know if it is possible to obtain this from a general point of view because precisely, it raises a very personal question: "What does a friend represent for me? " There is no universal answer to this question and therefore no universal solution either.
In my opinion, it would be nice to let everyone answer this question, with perhaps, yes, default options in the administration part, but also and above all, to let choose finesse for each profile would be ideal.