a disadvantag is that forums need moderators and require a certain amount of 'nannying'.
due to the natuer of forums (i.e joe public can sign up and post) you have to monitor what people say - especially on offical support forums, this is because certain topics may be out of bounds, (hacking / P2P filesharing / drugs) because offering support on those sujects may be incriminating to the people using those forums and to the hosts.
scripts can be written to filter swearing, which stops people swearing at each other. however scripts following certain rules are not always the most intelligent things.
for instance it would b a poor use of the word to call someone a retard, (so you'd filter the word retard) however this would cause problems when writting a post about the plight of retardation.
(hope that gets you started)