Very simply the problems include background illumination and harsh shadows. If using multiple flash setups the additional problem of mutiple specular highlights occurs. With light falloff according to the inverse squares law, when the foreground subject is well lit, the light from a single TTL flash is not enough to light a background more than a few cms away, resulting in black unnatural looking backgrounds. With multiple manual flash setups, one can control the distribution of light much better. I have thus far avoided flash in macro shooting and am just testing out a setup that Dan helped me with on a few other threads.
Originally Posted by glbeas