A guy on reddit who said he was a medical student (and sounded like he really was) said that the nutrients may be fine, but the digestive system needs solid food. Patients stuck on liquid diets tend not to do well long-term, and they try to get them on solid food as fast as they can. He had a technical explanation I don't remember well enough to repeat.
I asked how the Soylent guy would do if he converted the stuff into some kind of solid food bar, and the med student said "Probably much better!"
They've done studies yes. In Anatomy & Phys in college they noticed that recovering soldiers recovered mentally far faster from the solid food equivalent of a nutrient shake. In some cases, it was pivotal to survival.
From the subject post: "I made a rather significant change to the formula... I've replaced half of the maltodextrin carbohydrates with oat powder, which ... dramatically increases the fiber content ... I underestimated the importance of fiber in a diet, and went from consuming 1.2g / day to 40g / day."
I asked how the Soylent guy would do if he converted the stuff into some kind of solid food bar, and the med student said "Probably much better!"