Very true...the GPS trackers won't work underground at all. Quicktrak has some functionality underground, sometimes.
The thing is though, with either system, you will see the last known position (ie the entry to the underground carpark etc.) Once inside, the car can't dissapear!
However, at least with the GPS systems, you have coverage everywhere on the planet, with quicktrack (or anything esle that uses triangulation), once the car is out of range, it is gone...