1) The drow version is borked into complete non-functionality but is the subject of other threads so I won't wax on about it.
2) The rogue assassin version turns off every time you shrine or complete a quest. i.e. every 'rest'.
3) Despite that, the icon will stay toggled on through rests, fooling me far too often to admit.
4) The poison damage applies before assassinate. This means that if no one else has aggro - i.e. you are soloing, or are playing point man, the assassinate won't work.
5) I concede some hyperbole in the title.