I don't think that there is anything fundamentally unsafe about detachable towbars as long as they are checked and greased regularly and fitted correctly. i have towed a caravan for hundreds of miles with one and had no problems. The manufacturer has to submit them for rigorous testing to get them through EC Type Approval. But, they must be maintained and fitted correctly! As for interfering with reversing sensors, if fitted by a qualified supplier or as a factory fit, the wiring takes into account the towbar being picked up by the sensors.