Once corrosion is under control, is that danger actually higher than the risk of a natural gas fuel system leaking and killing a whole crew? Both gases are lighter than air, both form explosive mixtures with air (but ammonia-air mixture has much slower flame speed and slower pressure rise than natural gas - for which it makes up with its much higher toxicity).

In the end, gas leaks are always bad. But at least you can easily smell an ammonia leak.