HAHAHAHA. Yes it's dangerous. You might suffocate to death on the hill struggling to breath with snow clogged vents on your face shield. If you fall head fist into powder, you cant breath while you're buried anyway - face shield or not.
Actually if you're buried by an avalanche, in debris that sets up like concrete, you still have 15 minutes of air... I think you'll be fine in powder.
These helmets are retarded as fuck but they're not going to kill you. I WOULD be worried that the facemask thing would smash into your face if you slammed on it... A bx helmet they're not.