it is a priori highly likely to fail since we know for a fact that severe nutrition deficiencies can be due to subtle & misunderstood factors (see: the forgetting of scurvy cures) and that nutrition is one of the least reliable scientific areas

While this is true, I would expect that for many people, the main risk in Soylent would be one of overdosing on something rather than acquiring a deficiency. Soylent at least tries to provide everything that you need, while many people (me included) optimize their normal diet mainly based on criteria like cost and ease of preparation rather than healthiness. And even if we did optimize it for healthiness, your argument is essentially saying that we might very well screw up and acquire deficiencies even if we tried to ensure that we got everything that we needed.... and that argument can be applied to normal diets just as well as Soylent.

So anyone who uses the "there are lots of subtle ways of acquiring nutrition deficiencies and we might not know everything that one needs" argument against Soylent would first need to show why normal diets would avoid that argument any better.