Its generally pretty reliable, but if it fails, the car is not operative and it can be extremely expensive to fix. Sometimes it's just some speed sensors, but not always.
Its completely unnecessary on a 420. With its extremely tall gearing & second gear start, you'd have to be driving like a complete yob.
Agree , W126 with ASR is a no go unfortunately. Nobody can test this system to diagnose and parts are crazy price. Also to remove you would need a donor vehicle