Having worked with UBBThreads here and PHPBB on another site, I will take UBBThreads any day. Sone of the reasons:
PHPBB2 has too many ways of doing things. You can easially give a user permission to see a certain board, or add him to a group to see that same board. Normally I wouldn't complain about such, except that if a user is granted permission, it is a pain to quickly find out who exactly can see what across the board. UBBThreads limits permissions by groups only for the most part.
Unread message tracking. PHPBB2 likes to just forget it all, not allowing you to go in and read one board. UBBThreads allows me to read the entire Off Topic area, then come back several hours later and still retain my unread markers on other sections.
Trying to find a user in the Admin panel in PHPBB2 is a mess if you don't know part of the username. So many things in the admin panel depend on you quickly finding the user, and it's not a quick process 90% of the time.
I admit, PHPBB2 has some nice features, and it may be the way to go for a free install these days. But I really hope the 2.2 release fixes many of the existing shortcomings.