I have used witter for my last three tow cars.
On my last xtrail the bar came loose after 2 years, witter had provided the cheap "spring" washers for the job, the type that you should only used once, don't know the name for them.
But the bar professionally fitted had come loose at both ends of the main beam.
Although i again have a witter fitted, the bar now comes with the nuts with a plastic insert, and once again are galvanised(the last bar was cadmium plated).
But i am not convinced about the way the u bend for the ball is supported, via two 12mm bolts, just does not look substantial to me, a weak link, on a otherwise good bar.
So i will certainly be looking at other makes for my next bar.