I think the system we have works well, only try scoring is subjected and only if the ref calls for a review. The ref has to signal try or no try and any doubt will go to the refs original decision. As well the captains can have 1 on field review and if successful retain that view. Get it wrong and no reviews left, same as cricket reviews. All reviews are broadcast on the screens and video refs are miked up explaining their decisions (extra revenue as reviews are sponsored by KFC)