As UBB gets more replies its stability goes downhill. As newer versions of UBB have come out its stability has gone up. But still UBB's stability isn't as good as competing solutions like vBulletin. UBB is also easier to hack.
http://www.geocities.com/sbp7777/files2/bwhaha.gif UBB is a good solution if your not going to get many thousands of replies.