On the odd occasion that I forgot to close the valve for the pond return when flushing my S&G (), and my pond would go super cloudy from the waste, it is crystal clear again a couple hours later. I throw koi clay in, so that could be helping, but I feel that the S& G just recleans the water.
Also, I had an old sub pump running a little fountain a few years ago, that failed and leaked a slick of oil all over the water surface. I had no idea what to do, so I turned off the bottom drain and sent all the water through the skimmer to the S&G. Within an hour the oil was gone. I flushed the S&G a couple days in a row and never suffered any ill effects.