Never(!) solely rely on an HMI input if there is a potential for danger. Always add safety devices to protect people, and additional PLC code/sensors to protect equipment. For example, if you lose communication with WinCC in this case, your conveyor will be stuck on with no way to turn it off if WinCC button is the only source of turning it on or off. In this case, at least a hard-wired pushbutton to turn it off would be necessary to have to ensure the conveyor turns off.