Bossfights can be pretty chaotic, a lot of extra sounds are playing at once. Besides the boss’s own roaring, there is usually a lot of extremely loud audio queues to notify the player of his swings and agro patterns. On top of this there is the bosses music. All of this combined either makes it very difficult to hear an ambush sound, or outright replaces the ambush sound, and usually ends up with getting instakilled in the middle of a bossfight with very little warning. This problem isn’t so serious on bigger maps like Against the Grain or Athel Yenlui since the ambush mechanics seem to spawn them further away. But on maps such as Into The Nest (the later half before and during the Rat Ogre Pen), and Screaming Bell (usually around the poor folks pig pen and marketplace), mobs can appear within striking distance of the player, and kill them even before the ambush is initiated (usually with a coordinated scream). Yes, there is a backstab sound, but it’s very difficult to notice over the constant audio queues warning of bosses overhead attacks, block ignoring attacks, or agro change.
I think ambushes during bossfights would be a lot fairer if some sort of mechanic were in place where the ambush sound also reduce the volume of other audio queues. Also in general, the ambush sound doesn’t always seem to work even in normal gameplay, and it could be that other audio queues such as Chaos Warrior changing agro or overheading is also either louder or replacing them here.