With Agora you can create unlimited number of hidden forums for unlimited number of different groups. I can't imagine my forum without that option. If you need groups created by users, you can buy commercial component AgoraGroups. Agora can be also integrated with many Joomla extensions (like CB Joomsocial, jSocialSuite, UddeIM or Joomfish). If some extensions, like Group Jive, will not work with Agora, ask creators why they do not want to make their product working together with Agora. I asked developper of Group Jive about integration with Agora, he answered - only Kunena. So it's not Agora problem.
Why Agora? Because of groups with access , awesome support (answer from team in minutes, help with your live site by their hands), extended editor, users map, extended attachments system, Joomla Modules can be used inside Agora Component (there are standard modules positions where you can put anything you want - together with login module, which you are missing in Agora.
Example of login module on top Agora:
battlestations.eu/index.php/forum
And finally - design of both components... Kunena just looks like.... Kunena...