Nitrous backfires are most commonly caused by human error, meaning that dumb **** shouldn't be using oxidising agents, at the drags, a nitrous backfire is generally caused when someone stalls their car and forgets that they have to turn off the nitrous, on their solenoid operated nitrous system before starting the car, now the solenoid activates the nitrous at what ever throttle position it is set to inject nitrous into the motor at, which at the strip is full throttle, and the poor basterd in his haste to restart his shitta puts the foot flat forgeting about the giggle gas, and when the motor fires it has a belly full of nitrous and pretty well explodes resulting in you no longer having a bonnet or intake manifold. the rest of nitrous caused from a bad setup or running to much nitrous for the given amount of fuel, or running a "too" big of a shot of nitrous and bending or breaking rods.
It is not the product, its the application and the set up that causes blown motors and wallets when using nitrous.
*Note NUT347 Has actually been around the block a few times, most recently in an awesome Mq short wheel base patrol with a small block ford engine