The odds of finding out that something like barley and chicken are especially carcinogenic are a lot less than the odds that Soylent or Paleo or some other fad is frankly unhealthy.
Ignore the bulk of nutrition "research" which is after all, crap, and focus on the few things that are relatively sound. High fiber, healthy weight, good blood levels, get your nutrients. It's not that hard.
Couldn't it be argued that it's "relatively sound" that as long as you get a good balance of calories and macro and micro nutrients, which is exactly what Soylent provides, that you are on a good path?
FWIW, on the topic of healthy weight, the Soylent forum has many anecdotes of people who have used the product to lose (or gain!) weight because it allows them to easily count calories without having to spend time scanning products into MyFitnessPal every day and weighing all of their food.
The idea of "healthy weight, good blood levels, get your nutrients" is the goal of every diet I am aware of. But as for what goes in your mouth to achieve it, there is a plethora of conflicting ideas. I totally agree that the bulk of nutrition research is crap, which is why I put a barley and chicken diet, vegan diet, paleo diet, and Soylent diet in the same "beta testing" category.
Ignore the bulk of nutrition "research" which is after all, crap, and focus on the few things that are relatively sound. High fiber, healthy weight, good blood levels, get your nutrients. It's not that hard.