I am pretty sure IWT also breaks stealth because it enables auto attack, so you most likely can't use openers with rogues unless the rogue is the leader (since you don't need IWT on leader).

Rogues are squishy, hard to manage combo points and energy, has cds as gap closers (sprint, vanish, blind doesn't work great when multiboxing).

It is just the worst class to multibox, if you want to have a rogue at lvl 60 it can be worth to have one but it is the weakest link of the multiboxing group.

It is also harder to play a mixed team of ranged+melee because then you have to make sure melee goes in melee and ranged stays away, which might mean pressing some additional key before every fight. More likely to get stuck with melee in something or face the wrong direction with ranged. You probably want to have IWT enabled on casters but not use it unless you need to face enemy or talk to some npc but on melee you want to use it all the time.
Also makes burst uneven since ranged can start right away but melee need to go to enemy, can get stuck in slows/roots and use different kind of abilities (shaman have the best burst in pvp since it all happen at once so it is unhealable).