The main issue with most ninja's survivability is overestimating our dodge ability (we get a permanent movement speed buff) and pulling too much aggro too fast (we don't have any aggro management skills, that coupled with high skill speed is bad news). As others have stated the animation lock from assassinate is also a killer for our kind, it's easy to get locked into a thought process and use it at a bad time resulting in you being locked in an AOE.