The biggest risk is the tyre fitter catching the sensor part when removing the tyre (although most seem to be wise to this now)
The bigger risk is the metal caps that these have as standard corrode onto the valve stem then snap the stem when you try to get them off!!
As you don't need to check the pressures manually you don't know its an issue until its too late!!
Vxl's replacement valves are £32 - £40 each on line and the service kit (replacement valve insert and rubber 'O' ring) is about £3 plus its 'self learning' so if you only have 3 valves fitted it doesn't come up with an error it just monitors the ones that are there, plus if you add a new one/ replace a removed one it automatically adds itself to the system, no need for computer adjustment - this is based on the Vectra C/ Signum, which due to standard Vxl half ar5ed look at life, some cars only have it on 2 wheels (front) some do all 4.
Main problem is you cannot manually check the pressure you don't know if its actually working until a problem occurs and you cannot change the threshold - tyre pressure is 41psi it warns you when it gets below 37psi.
Although the Vxl system is quite good and fairly simple I would put money on the Frog version to be a real PITA