The downside of using an off-camera flash is making sure that you’re using a flash that is no faster than your shutter speed. This can be difficult, since electronic cameras will override you if the flash is on, and older cameras may get a blackout if the flash is faster than the shutter.
Using an off-camera flash also makes it difficult to coordinate the flash going off and the camera taking the picture. If you have a shutter speed that is too slow when you’re using an off-camera flash, you’ll get the hideous black bar. This is caused when the shutters haven’t opened all the way, and the flash goes off! Since the shutter isn’t open all the way, part of the picture is blocked, and all you’ll see there is a black bar.