Gear? Level? Your star level? Sideous and Lumi are not that tanky, expecially sid.
His thermal detonator is often terrible as it takes time to go off by which time the match is already decided, he has no worthwhile abilities, his crit chance is 65% at max rank of his unique. If he was teh leader he had a 100% crit chance and 25% increased turn metre on crit, which could be a deadly combo, but his leader ONLY works on stealthed characters so is useless for most of the team unless you have a full teebo stealth comp, even then you have to get lucky with going invisible.
Not very many people have bothered to level him or farm him, so on paper he sounds terrible, 6k crit damage isnt even all that special compared to qui gon usign his assist ability on a jedi team, or maul attacking a jedi opponent.