That sounds right, as the aftermarket control arms are generally used with a camber kit to correct negative camber that occurs when lowering a car. So, if you used them with 0 camber, you would end up with positive camber so from the rear of the car your tires would look like \ /. This could definitly make the rear of the car unstable since the rear wheels would tend to dart in either direction rather easily as you discribed. It can also wear the outside edge of your tire in the same way that negative camber wears the inside edge.