thread on is ALWAYS better, u want the centerline of the brake in perfect center line of the bore, if not gas exists differently perside and can effect accuracy, of your bore isnt exactly center of your barrel, or the machineing of the brake is off this can happen..
do they work? yes the quality of todays factory rifles and aftermarket brakes is pretty damn good.
are they they best option? no, a properly threaded and clocked brake is always better