Kelly,
Skip is correct in that improper air/fuel is a major reason for any forced air induction based modification to fail. The real culprit is "Detonation" which is the killer of ANY nitrous/turbocharged/supercharged applications. Detonation is the result of inadequate fuel delivery, too much ignition timing or a combination of both.
There is also what I like to refer to as the "******** syndrome" where the owner comes out of the movie theater from watching Fast and the Furious and decides to go home and drill out his nitrous jets or other stupid "in home Sanitation Engineering brainfarts", blows the motor and proceeds to tell everyone how terrible nitrous is.
From personal experience of over 20 years of racing and using nitrous, I have never hurt an engine using nitrous. My Viper has been using nitrous for over 5 years and runs consistantly in the 9's at over 145 mph. The motor compression and leak checks as good as it did when I first checked it.
On a different note, using my own Viper engine as an example, I have about $ 4,000.00 in modifications to the engine from when it was stock. It has been making about 800 RWHP and 1000 RWTQ for several years. Do you have any idea how much one would have to spend to match that power output and reliability from a different method? Add another ZERO behind the 4 in 4,000.00 and you will be getting close. In reality, If my Viper engine blew up today, I would give it a supreme burial ceremony in the backyard next to my pets, salute it for incredible service, and call Dodge to order another one!
I have got my monies worth out of this combination of strong stock components and a quality nitrous system....several times over! (if the engine did break, chances are that the cylinder heads and all of the nitrous components could be re-used, therefore only requiring my investment in a new basic engine..LOL)
I'm a ******** racer, the kits that I sell have an excellent record. I have never had a customer call me and say that the nitrous system did not perform. I know customers who have sold their Vipers and my nitrous kits several times over and they are still working properly after changing hands several times.
Over the years of Viper racing and watching the Viper High Performance Industry grow, more and more tuners are using nitrous, even using it as an additional modification to other modifications like turbochargers and superchargers. Nitrous is still king at the track as well.
With that giberish said, to answer your question about longevity, my kits are designed with that in mind. Specifically with propane usage the torque gain is tremendous especially when comparing performance tests to a typical gasoline wet kit torque output. The usable RPM power band when using nitrous is lowered about 1200 RPM when using my systems, this means lower shift points. This in itself will aid in longevity. Also keep in mind that with other types of power adders some of these do indeed induce constant wear on components. Nitrous is only "wearing" if you will, during the short bursts of time that you are using it. So, longevity in my opinion is a proven statement based upon engine performance and miles or races campaigned over the life of the engine. No power adder is guaranteed to make your engine last longer(and usually the opposite is true)...that kind of majic stuff can be bought at Autozone and poured into the engine from little plastic bottles....LOL.
I hope that this helps. Feel free to search these forums or contact us via EMAIL with any questions or for a list of customers who would love to tell you about their nitrous systems.
Thanks
Tom