Although not quite as extensive as phpbb or other forum solutions, Kunena has come a long way in its relatively short life span. The level of difficulty is medium I would say as you do need to know how to navigate Joomla fairly well to set up permissions correctly with User Groups and Access levels. Once set up, it works great though.
It also scales very good. If your site is slow, it is probably not Kunena that is slowing it down.
The only down side is that since it is a free developed component is that support can be somewhat limited outside of the standard features that it comes with. So, if you are looking to style and change internal features, you will either need to hire someone to do it for you or know enough coding skills to do it yourself.
One feature that is missing is the ability to see Unread Posts, which my users seem to constantly remind me about. Last but not least, there are not a whole lot of templates out there for K2.0 relative to other forum solutions.
Nonetheless, give it a little bit more TIME and Kunena WILL be the go to solution for Joomla forum software.
All I had to do was install, set the module to a position and pages and then get a Disqus API key for my site.
Otherwise, it style looks great without any effort.
The level of difficulty here is very low and I'm a relatively new user when it comes to Joomla! Thanks so much for this great tool!