Having used the Bose X for three years, I do not buy into the belief that the Bose ANR "masks" sounds, because it doesn't. In fact, I was amazed on the first few flights how many more sounds I could discern once the overall roar of the engine was attenuated. Transmission whine, tail rotor buzz, the more subtle sounds of the main rotor working in the air, all suddenly became apparent.
If I can hear the alert beep of a Garmin 195 over the sound of a Lycoming HO-360 pulling 27" MAP in a 60-KT climb, I'm pretty sure that a stall warning horn or the sound of a 134% overspeed would be painfully obvious. (I have no idea what make of ANR headsets might have been used in these incidents.)
Now I can't speak for all ANR systems, but for me, at least, the Bose means I am more likely to hear the important details. I would hazard a guess that anyone who claims the Bose ANR creates a hazard has very likely never used a good ANR headset.