Harnesses really need to be installed properly to be ultra safe too, and I bet there are many people who dont install them correctly....this would be the main reason that they are frowned upon by the po-po.

The two important things to remember are:

1. The belts cant be attached at more than a 45 degree angle relative to the back of the seat. If they are, its a great way to compress your spine in an accident because it pulls down on your shoulders rather than holding you back into the seat.

2. If they are bolted into the rear parcel shelf, spread plates are required to fit underneath the shelf, NOT just ordinary nuts. These are basically plates with a nut in the middle, which "spread" the load over a larger area and prevent the bolts tearing out in a big accident.