We also have to adulterate that methane with bitter smelling agents too warn people of the danger when there's a leak. The line into the house is also limited by a regulator to ensure the pressure is very low. If gas builds up in a battery, it's either going to leak out slowly or build up and leak out all at once.
Very much not an equal comparison.
The methane is almost always piped in to be burned, and that can easily create odorless carbon monoxide. And the smell is not foolproof either. This does routinely kill people and we keep doing it. The jurisdictions that are banning it are doing so because of environmental reasons, not safety.
What the other poster said about the risk of releasing cyanide during overcharging is not true.
Cyanide could be released only at high temperatures over 300 Celsius degrees.
During a fire, there are many other things in a car that can release toxic fumes easier than a sealed battery.