It depends on the use case. Flat forums are vastly superior when it's about having a discussion (singular) rather than a bunch of splintered bits of information being shared.

It's worth remembering that Usenet had threading -- it's nothing new.

I would argue that a chronologically linear thread with no up- or down-voting can provide a level of focused and in-depth conversation and on-topic debate that is simply not possible with branching threads nor instant chats.