A friend raised a good point on facebook that I thought you might know the answer to: what's the difference between soylent and medical-grade food substitutes (like, what we feed to people who can't feed themselves, to keep them alive until they heal)? Because she was wondering whether there are uncertainties about using soylent in the longterm (potential nutrient deficiencies, or something), and if there are, why don't we solve them in the same way we solve the issues that might arise for people in comas etc.
I expect that Soylent is probably similar nutritionally. However, since it isn't going down a feeding tube, I suspect that Soylent tastes better. Even so, I can't imagine that it would be the optimal approach to eat the same thing every day, regardless of what it is. We evolved with varied diets, so I would assume that our physiology is optimized for variation.