They have the advantage of being cheaper, and much smaller. The disadvantage is their range is normally only a few miles, so they need to be 'hunted down'. In recent years the most popular technology for remote tracking involves a combination of GPS receiver and mobile 'phone. It just needs a 12 volt supply, from the car battery. So even a small car's 35AH battery is often sufficient, without installing more complex power-saving devices.
Some companies obviously specialise in hardware more suitable for covert installation.
The lowest cost solution may be a combination of GPS and mobile 'phone. The total cost is then that of the hardware, and the monthly rental of the 'phone. There will be an additional cost, for each 'phone call made, but this will be small if the number of calls is small. For many anti-theft or security applications this may be sufficient. If the vehicle owner wants to know where the vehicle has been, each minute of each day, this can achieved with memory in the hardware.