For example:
https://discourse.soylent.com/t/phytonutrients-and-all-the-o...
https://en.wikipedia.org/wiki/Resistant_starch
http://www.webmd.com/diet/guide/phytonutrients-faq#1
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4287321/
https://www.ncbi.nlm.nih.gov/pubmed/26757793
There are all sorts of compounds in natural foods that science hasn't even begun to analyze in depth, but which humans have been eating and coevolved with for millions of years. What's the effect if you take them all out of your diet completely?
Soylent's strategy, understandably, is to put in all the things that we think we already understand and know we need, but what if that's only 10% of what the body needs long term? How do you quantify the risk from unknown unknowns?
Also, the comparison isn't really between Soylent and oranges alone, but Soylent and a normal balanced diet, of which fruit is one component.