Make sure they have a "beep" feature that notifies you when someone keys a radio on your channel/code
And any clown on the ground with a CB and an illegal linear amplifier will trigger your system before you wanted to... uncool. A little more discrimination than that would be needed.
The code featue on the FRS would make the odds of that extremely low. Same number of channels, but your beeper only goes off when another user of a compatible set sends the right code.