The main problem with sugars is that they have a high glycemic index, which means that they break down quickly and rapidly release glucose into your bloodstream. That's bad for several reasons. One is that it causes a strong insulin response, which can increase insulin resistance over time, leading to diabetes. Another is that it will give you a burst of energy initially, which will then fall off, leaving you feeling tired. Carbs with a lower GI will keep your blood sugar at a steady level.

But actually, looking again at the soylent recipe, he seems to be preferring maltodextrin as his carb source. He was probably mislead by the fact that it's a "complex carbohydrate", but in fact, the GI of maltodextrin is higher than cane sugar. In fact, it has basically the same glycemic index as pure glucose.

So I retract my previous statement. Unless you replace carb source in soylent, SlimFast is basically just a better option.