Some photographers with big equipment budgets love really fast lenses like f-1.4, as these lenses reduce the amount of flash required, allowing for more natural light photography. In addition, other challenging situations require really long lenses, like 200-300 mm. If you are a photographer with full frame sensor cameras and you can afford really fast lenses, these lenses are really huge and arm busting heavy. These large and heavy lenses, when mounted on a camera on a flash bracket, add significant stress on the bracket flip mechanism.
The stress that these lenses produce on the camera mount and associated bracket parts are cantilevered, thereby causing significant force. All flash brackets would prefer a balanced force on the camera mounting and actuating components, but really heavy lenses upset the balance point, adding significant front rotation force. These forces will most likely not break the bracket, but they will affect the function of the bracket, sometimes making it awkward and difficult to flip.
Therefore, it is best to use lightweight, small lenses (f 2.8) on all flash brackets.