The wisest thing ever said on television was in an old margarine commercial: "It's not nice to fool Mother Nature."
Eating Soylent exclusively, or as a high percentage of food intake, is unwise, and the fact that some people seem to be getting away with it, unscathed, should offer little reassurance. As others have remarked, we have no way of knowing if we know all the essential nutrients Mother Nature wants us to have. And we may not ever know, because nutrient deficiencies can take years or decades to appear. Vitamin B12 and vitamin E are examples of this.
I recall one lecturer talk about a vitamin or mineral deficiency that was discovered by medical science only because a woman had habitually eaten nothing but an egg on toast for 20 years (cannot remember what it was). And then there are other deficiencies that have come to light much more recently, when hyperalimentation solutions lacked something that humans had never before suspected was essential (I'm thinking vanadium, but again am not sure).
On top of this, I would additionally be concerned about the very naive medical reasoning that Soylent's inventor evinces. On this page http://robrhinehart.com/?p=424 he wonders if his family's fondness for tomatoes reflects a salutary effect of lycopene peculiar to his family's genetics. Nothing is impossible, of course, but the unstated assumption, which appears often in Soylent discussions, is that human appetite for specific substances is correlated with the body's need for, or benefit from, those specific substances. Although true for calories (we get hungry when deprived of calories), in general it is balderdash -- go read about pica and cissa. (You'll read, for example, about iron deficiency causing a massive craving for ice, which is, of course, iron-free. Iron deficiency also causes craving for tomato seeds, which are poor in iron; maybe the inventor's family is iron deficient.) And then you can go read about B12 deficiency (called pernicious anemia, because it was uniformly fatal). Those people had aversions to meat, when, in fact, meat was the food richest in the nutrient they were missing.
Life is an exceedingly complex and subtle machine, and it's risky to think that humans can re-engineer macro processes that are built into us at the deepest levels. If you consume a lot of Soylent and get a disease unknown to medical science, I will be very happy, because we will have learned something about metabolism, and minimally sad, because it was a choice you didn't have to make.