Depends on what type of backstay mount you mean. There are two different styles. The ones like Scanstrut seems to work well and I know several people that use these types and like them.
Then there is the kind that rides on the split backstay. The one person I knew that used one hated it. The problems were twofold. First, as you sail, the tension on the backstay loosens. This allows the radome to turn to one side or the other. The result was straight ahead wasn't straight ahead, but rather off to the side of the display. Secondly, the wire has to be led down the backstay. It was always interferring with the ability of the radome to level.