You would be unlikely to loose weight as they dont contain all the vits and minerals you need your body would most likely go into starvation mode and use up muscle instead of fat ( and your heart is a muscle so not good ) ...... slimfast is designed to be used along side a healthy meal so it would not work , and I guess it would also not keep you in ketosis
high sugar content would lead to lack of ketosis and severe hunger
possibly diarrhoea as sugar is an osmotic.
Lack of protein would lead to catabolic state in the body you would start to break down muscle, above and beyond "normal weight loss rate", for calorie use, leading to potential heart problems, weakness, lethargy.
Lack of minerals means you put your overall health at risk, same with the vitamins