there are also things that are out of your control, solenoids sometimes sieze up, the nitrous solenoid can freeze if bottle pressure is too high, poor fuel quality causing detonation, etc. you must have alot of faith in the installer and tuner, in saying that i have seen guys fit there own kits, have no guages for gas pressure and fuel pressure reference and still have no issue, mind you the cars werent neer there full potential.
this system is for motorsport use only in a conrolled environment. not recommended for street use. it makes cars fairly angry.
Also ends up expensive as a 10lb bottle costs $120 to fill. thats a dollar a second.