The idea is great in specific context. People have been living on single food item in multiple occasions so far. CBB to find references for all now, its easy if you need them but there are known cases of
1. Potato only for 6 months (http://goo.gl/XqxMx)
2. Bananas only for months (i.e. extreme '30 bananas a day' diet)
3. Meat only for a year (Vilhjalmur Stefansson)
4. Water only for > year (in extreme obese case).
5. Protein powder only (i.e. Last chance diet, case to the point of what could get wrong. )
6. Synthetics only (I am sure Kurzweill probably experimented with this with his 200++ pills per day).
7. MC Donalds for a month
... and so on
Its important to note that inventors are young people. Young people are very resilient - they also have full reserves of stem cells that are used to repair damage. Soylent might speed up this process because it certainly doesn't contain 'everything body needs' (because we don't know this at the current technological level) in which case one theory is that body will use its triage system (Ames) to redirect resources to the systems that are most important and shut down or reduce output in less important systems for immediate survival (such as reproduction). More appropriate would be to say 'everything body tolerates for a period'. Its certainly far better move then eating only pasta or rice for entire day.
One may hope that whatever is the eventual damage that could be done to you by use of this food surrogate is going to be fixed by the future medicinal, yet to be discovered, techniques.