If your someone who has to have their Bivvy all shut up when you go to sleep or a tiny letter box gap then a Bivvy heater is just asking for trouble ,
I had one bought for me last year ,but as I never have the front on my Brolly or Titan its never going to be a problem , and if the front does go on its because im facing into the rain and that in its self means its warmer so no need for one .
So it all comes down to whether or not you have the intelligence to use one correctly , IMO they are not dangerous , The danger is in the way they are used.