We've only begun to start understanding some of these diseases (like Pellagra: http://en.wikipedia.org/wiki/Pellagra#Epidemiology), and we don't really have it all nailed down yet.
But we do know about nutrients and have a good idea of the proportion of them for the avg human.
I think my main problem with the backlash is with the argument that boils down to "We don't know, so let's not risk it". I don't see why we can't try it, and then monitor and react. If I started using Soylent and then 30 days down the road I was feeling ill, I'd probably stop using it first thing while exploring other causes.
We really, really don't. You could be excused for thinking we've got it all figured out if you listened only to news reporting on nutrition studies, but there's so much contradictory information out there that it's difficult to know which end is up.
For example: Soylent, according to the story, is partially fish oil. Did you know that there's a study out of the Fred Hutchinson Cancer Research Center that strongly suggests that fish oil supplementation increases the incidence of aggressive, fatal prostate cancers? What was once a nutrient panacea is likely now contraindicated in men.
Does this mean that Soylent shouldn't be using fish oil? Hard to say. I doubt that even the creators have any idea.
I welcome doctors, researchers, and people who know what they are doing to experiment with the human body. But when a computer science major armed with nothing but a few textbooks in body chemistry claims that he has "figured the body out", I am going to assume otherwise.
Does he have any cooking experience? What is his medical background?
Its not so much that "we don't know", but "the creator of Soylent clearly doesn't know", and yet he wants us to believe that he does.