From the Known Issues list:
The reason its performing poorly is because at some point on lama it (accidentally) changed from triggering when YOU hit a mob to triggering when the MOB hits you. I am assuming they will fix this back (sooner rather than later) and as a result the enhancement should be adequate at that point in time.NEW: The Rogue Assassin's Nimbleness core enhancement is not providing a benefit on-hit, but does provide a benefit when hit.
As for the rest of the feedback, yes sneak shouldnt be quite as easy to break, but not sure I want to see it pawned off on concentrate. Thats a cross-class skill for rogue. Maybe they could just make it apply a debuff thats -20 to hide/move silent for 6s after being hit, rather than fully breaking you out. That somewhat mirrors pnp, and allows someone with superior skill to remain hidden under duress. To avoid being infinitely hidden with sufficient score, just make it stack up to once per second. If one of your scores goes under 0 due to being consecutively beaten over a few seconds, you unsneak.
And re: Killer, would you feel this offered more if it faded one stack at a time rather than all at once? That has been asked for numerous times and ignored, but the practicality of it is that when playing it needs that functionality or its an AP sink. As it stands its a 0-1 rank buy, which is unfortunate.