Sorry, I disagree.
EVERY year, common sense fails some and they pay the ultimate price.
A bivvy heater in the bivvy with the door closed is asking for trouble, no matter what gas. A fire in the bivvy, or carbon monoxide, and somebody gets severely injured or dies.
Carbon monoxide is caused when the fuel does not burn properly, whether it is gas, coal, wood or oil.
Direct quote from the NHS:
Causes of carbon monoxide poisoning
Common household appliances used for heating and cooking can produce carbon monoxide if they are not installed properly, are faulty, or are poorly maintained.
Appliances that can cause carbon monoxide include:
gas boilers
gas cookers and clay ovens
gas or paraffin heaters
wood, gas and coal fires
portable generators
Using barbeques or camping stoves inside, and turning on vehicle or lawn mower engines in your garage, can also cause a build-up of carbon monoxide.