I have tried to do a search before posting this.
I am pretty disappointed in how sub forums are displayed on the main page. It's like they are an after thought. Sub forums are VERY powerful and to have them enabled under main headings means the overall layout is cleaner. The issue with UBB is the fact you can hardly see the fact there's one actually there. Until you click the "parent" or main thread it's not obvious.
It might be an idea to address this going forwards in your next update. This is where the likes of IP.Board just smash UBB.
I have both boards for different sites I run and prefer UBB on the whole... but with sub forums being such a massive deference I prefer IP.Board for this reason.