I never played an Assassin until now and do not recall ever playing a Rogue in epic levels at all, so putting a pure rogue assassin (my main, Completionist) through epics is a new experience for me.
And in every party, raid or otherwise, it has been a disappointment.
Playing Assassin solo is great fun, even if slow. As of right now, she is level 25, and soloing Sschindylryn quests EE was a perfect test of her abilities. End fight in each of three Houses was a drawn-out affair as I would sneak in, kill one of the guards, perhaps pull couple more out into the corridor, repeat until the boss was alone. (Don't see how this could work in Portal Opens though) But in every party I was relegated to being a trapmonkey. By the time I sneak up on a mob, it is already dead, and my contribution to boss fights was minor. Might as well go Mechanic route and optimize for traps.