the amount of updates that is required to the database in order for it to work, access often can't cope and the database will become corrupted crashing the forum, the only option then is to download and compact and repair the database in access to get it going again.
Also as access doesn't properly delete entries from the database, the database size will grow and grow, thuse slowing down the forum.
I came across one site the otherday that had a 29Mb access database, this size was mainly due to the active users table and items not being properly cleared out by access, after compacting and repairing the database, the size went from 29Mb down to 7Mb.
Unless you have a very, very quite board I would keep the active users list turned off in the access version.