Did I hear my Name?
They are all safe when installed correctly and used as most define.
Never engage nitrous until you have reached a minimum of 1500RPM. I prefer 2000. Things need to be moving through the manifold so you do not let the nitrous "pool" in the manifold. It causes a backfire. Remember it is not gas you are injecting, it'* nitrous in it'* liquid form. I used a purge to make sure it'* all liquid and your not getting a sputter which causes a backfire. When you stop injecting nitrous it is equally important to not just let go of the throttle abruptly or again, nitrous pooling can occur. Once you engage the nitrous keep your throttle firmly to the floor thru the gears. A "flinch" or sudden let off of the throttle creates a backfire.
Dry setups are really "ancient technology" and what gave nitrous a bad name in the beginnings. Dry nitrous setups can only help burn what gas the car normally delivers and when all the nitrous can't be utilized that causes backfire as well. You need to use copper or iridium coated copper plugs or the tips can burn off quickly and again, backfire. Seeing a pattern? Nitrous backfires can be quite violent as the gas and extreme oxidizer upchucks back up the intake...or whats left of it. They can be violent enough to crack intakes,fuel lines, cripple fuel lines, and even throw a piston.
Dry setups also make for easy engine damage. Being that you are using an extreme oxidizers to merely help what gas is normally delivered you are creating a very dry hot engine. Wet setups allow for more gas to be delivered at the same time as the nitrous and keep things in "balance". Zex systems are just a bit more than others but give you alot more efficiency and safety. Other brands are starting to follow suit with this kind of system. They are triggered by your TPS when it registers a full throttle response. As well the Zex wet systems have a computer chip that graduates the nitrous valve according to two factors. Both gasoline pressure being delivered from the fuel rail tap and existing nitrous pressure in your bottle. Both are critical measurements or.... you get backfires.
Not sure where people started think Nitrous was flammable. Maybe the similar name of nitro (methane)? You can even breath it. (If you know where to get it un-sulfated)
Or a friendly visit to the Dentist.