We could go through all the unlikely scenarios all day long.
I think it has long been proven that a vented system is inherently safe, the worst case I've ever heard of is that the system vents over, softens the plastic cistern and then hot water can be released to the inhabitants below.
It has also been proven that an unvented cylinder can explode when it's over pressurised - https://forum.nachi.org/t/exploding-water-heater-in-phoenix/30989/2
There are always risks no matter what system is used but I know which one would be preferred in a failure situation.
I think it has long been proven that a vented system is inherently safe, the worst case I've ever heard of is that the system vents over, softens the plastic cistern and then hot water can be released to the inhabitants below.
It has also been proven that an unvented cylinder can explode when it's over pressurised - https://forum.nachi.org/t/exploding-water-heater-in-phoenix/30989/2
There are always risks no matter what system is used but I know which one would be preferred in a failure situation.