Post ordering is a bit more tricky, and confusing the visitor.
For example, if you sort on oldest in most situations and forums the "parent" post is oldest, so it shows at the bottom (or top)
but the latest reply is actually three levels below it and "posted 1 second ago", and kinda disappears.
Clicking on the frontpage of 'latest post', might attempt to jump to that anchor, but what they see is an old post with a tree of comments spread of various dates. This confuses them and they start scrolling instead of reading. a/b testing cases I have done in the past resulted in less time spent on still screens vs scroll screens. Total time spent on a thread is 29% less because people feel they've already read that thread (just not the last couple of mini replies to that thread) and tend to just scroll away from it trying to find a better top comment (which is what really works for Reddit for example, top comments float easier to the top, tree-leveled blah replies tend to disappear until they get upvoted a lot more).
Another argument could be in favor of flat thread view versus threaded thread view is the 'wrong post reply', they want to reply to the thread, submit to the end, or they submit to the first reply they see. Their content appears to be an individual thread-reply's reply, but in fact it should have been a top parent reply to the original post of the thread. Or of course, the other way around. They read a threaded reply, click reply, and make what appears to be an offtopic comment to a 6 days old post near the bottom.
In threaded view the Quoted text usually isn't included, to keep things 'neat', but in flat view it is usually included, and makes more sense. The former will create confusion if people start changing the sorting of the thread.
There used to be a bit of a meme for BBS sites about Re: Re: Re: Re: "lol, yeah nice" just because it's 4 levels deep. Imagine allowing 12 level deep replies or more. A quick 'tnx' post would end up making no sense anymore. In XenForo I don't think the title is copied into the post reply, it's an old vBulletin and etc thing. So not really an issue. But still. How would you visualize this in an email, if you can't really determine if the reply is on or off topic, because you can't quote the content - which level is it quoting it from?
Another argument against threaded view is that people already put the UI/UX in their memory to navigate the site. Adding more hierarchies to this for the vertical, removes their focus from the content. While we geeks are a bit more techy, my father isn't. He rather have a list of posts to a topic .. this makes sense, and you at most have to remember you were near the end, or on page 2. In a threaded view, you have to remember where in the list you were, and hope no new content got ajax injected into it somehow in the meantime. Adding more to the human memory to memory the threads, content of posts they just read, etc to remember the vertical navigation on top of the board's own header, footer, sidebar, first post, read content, unread content, etc .. it's piling up too much.
the complexity also eats away with usability. If you allow a 6 level deep tree, vs just 1 or 2, .. for example how wordpress blogs sometimes operate, has an effect on how people can find their way in busy threads. Does it still fit on their screen? What about on mobile? No use having a triangle of white space on the screen, with 1 word cut off on the site, always requiring scrolling one way or the other. I know I wouldn't care one bit for those sites and would simply close the tab.
Threaded views, I have yet to find one I personally like, I really prefer at the most a single indenting of sub-replies. But I really prefer the flat-view.
