Game pad compatible

It has come to my attention that there is no gamepad that will work with emulators, specifically on a Mac op system. Is this true? Does anyone know of a game pad that will work with an emulator on a Mac? I'm told it is do to the way emulations run on a given system, and does not have to do with the gamepad product. If so, are there any emulators that will work with a Logitech or similar gamepad?