Multiboxing melee teams is terrible in all MMOs, but at least it's possible to do in WoW because of the Click-To-Move setting.
One problem with Click-To-Move is that it doesn't have a max range. (Though this is both good and bad.) It means that your characters will run far away randomly if they didn't acquire the assist target properly (which happens due to Internet latency if you switch target quickly). You can minimize this by extending the time between selecting a target and then using Assist and then executing the action. This makes melee teams have slow reaction time.
The other problem with Click-To-Move is about characters running in circles. This is because of mob aggro. If the mob is attacking a character that is using Click-To-Move, they will run in circles. You can minimize this by always making sure your leader has acquired aggro (e.g. with a taunt) before you use Click-To-Move on your other characters. This also making melee teams SLOW.
The third problem with Click-To-Move is that I hate when it's enabled on my lead character.
I think it makes it difficult to mouse-look with the right mouse button. This is a problem for me because I'm using a (proper) FTL setup where I can instantly switch to any character and lead the team from that new character when I want to. So having Click-To-Move enabled for any of my characters makes it annoying to switch to that character and lead the team from there.
The fourth problem with Click-To-Move is that it will only move the character to the location the target had at the moment you triggered Click-To-Move. If the target moves somewhere else, even just a short distance, your character will still just run to the original location and stop there. Since targets often move around a little bit, you have to use Click-To-Move frequently. Because of the problems listed above, this makes melee teams EVEN SLOWER.
(This said, my current main team is a max-level melee team of 5 paladins - one tank, one healer and three dps. But I intend to switch out at least the three melee dps to ranged dps.)