Problem with nested view/flat view hybrid
Nested view being the default just makes flat view not work. With a default flat view when conversations are going on people will use quotes. With nested threads people will just reply and it will be added to the nested thread. When you have both there is no way to keep track of what is going on with flat view because people using nested aren't going to utilize quoting and so there will be tons of posts that are completely lacking context.
This combined with the upvote/time posted determined visibility of posts will encourage people to reply to whatever nested comment that gets attention first and then everyone replying to that will further make flat view impossible to follow. That is completely ignoring how reddit style voting/nested threads promote circlejerks, and hide unpopular opinions. For example on GD there is currently a thread calling out the community for sexist attitudes. The thread is downvoted to hell, but there are enough people willing to defend it that it still gets visibility and there are like 20+ pages of discussion. Now if you had that same community using a Reddit style forum the discussion would be buried and get no visibility since sadly it seems like the majority of the community doesn't see how a comment like "'Woman' is life's easy difficulty setting." being upvoted as a popular opinion reflects poorly on our community and is really fucked up.