Most tie in with the stock alarm so you don't have to bypass it, then add resistors to arm/disarm it. Remote start is just finding the wires and splicing, the keyless just needs you to wire in the correct resistors to open/close the doors. Viper has a paper with the resistor values floating around the internet for our cars if you do it yourself.
IMHO what makes an alarm good or better is the expandability of it, adding sensors so the car can react more to different attacks that it may get subjected to, that's one area where the stock alarm is very limited. Just buying an expensive alarm without putting the extra sensors like glass/shock/tilt or even a radar module doesn't do much for you but you don't need all of the sensors.