Off the top of my head:
I like the format of regular forums. Vbulletin (like this one) or even phpbb. Look and feel kind of things. I think they're more flexible.
If I walk away to have a smoke or do something, I get timed out and have to log back in
Never able to find anything with the search function. On Bill Myers' forum, if I do a search for a post or something in the forum, I get results for articles but not something I knew I saw but can't find.
David Frey killed his forum when he went to Membergate. It wasn't the most active place to begin with, but weeks go by now with nothing there.
I guess a Membergate site hasn't stopped me from joining or going there if I wanted the info.
As a site owner, there would have to be very compelling reasons to invest in the cost.
Hope this answers your questions. If i can add anything else, let me know.
Larry Foster

PS. I look forward to seeing your project.