I never administered an UBB5 but I remember it being insecure and harsh on the webserver while content kept growing.
One large forum I remember (1000000+ posts) kept running into trouble and directories used to store forum data (like the posts) had to be logically adapted on the server to manage the load. Another smaller (yet cult-like) forum had 2 busy administrators who were out a lot, and the forum kept getting hacked somehow. They injected code into the frontpage (perhaps in the template files?) leaving it unusable. And without the cellphones and WiFi stuff we have today, it took days, sometimes even weeks, before the forum was restored to an usable state.
That said, looking through The WayBack Machine at my favorite old forums (go to https://archive.org
), I indeed like how simple and usable forums were back then. Back then they were my primary form of online communication, and it still is in a way.