We've just gone live with UBBT on our site, having used DCForum for the past three years. As expected, the main issue that's driving away a segment of our readership is the way flat works.
Here's a couple of screen shots I took that show what we're trying to accomplish:
In the DCForum, you can do a View All in threaded mode, which shows all the posts in thread order and indented so that you can tell visually where they come in the thread. Here's an example:
Threaded View All In UBB, when you click on 'All' to look at a thread, it comes up in flat mode. If you click on the 'Threaded' box at the top of the page, you get the subject lines in thread order and indented, but to actually read the posts you have to click on each one. Here's an example:
UBB Threads Why is this bad? Well, it's not a deal breaker for me, but it does limit my viewing options. I usually start by doing a View All in threaded mode and read all the posts.