"Do you have the intelligence to use one correctly?"
I'm sorry, but as far as I can see there is no such thing as use one correctly, there are simply too many hazards around.
We have the obvious carbon monoxide poisoning, but also anything falls on a bivvy heater and you can have flames that take hold in seconds.
To keep heat in, you will want to shut the door. That increases your risks to start. Then while the heat is on, you may well fall asleep. Is that a sign of carbon monoxide in itself?
A stove in the bivvy is a big enough risk in itself. I think every anglers groundsheet probably has melted patches on it from the stove in the foorway and putting a hot pan or kettle on it...
The answer is proper clothing, decent sleeping bag.
Finally, just as a reminder, and remember I know the guys concerned, it was that local to me:
https://www.anglersmail.co.uk/news/bivvy-blaze-horror-81088