Recs.
Updated
The initial sample size was small, the timeframe was short and no clinical study has been undertaken. Largely replacing your diet with a substance known only to be acceptable to a single individual for a short term is therefore inherently risky.
Business Insider featured Soylent, stating:
Rhinehart's claims are completely untested, have never been included in a clinical trial, and his diet isn't being monitored by a doctor. His self-experiment is ludicrous and most likely dangerously unhealthy.
Specs
Pros
Pro Important gut bacteria may need the other substances in food.
The bacterial ecosystem in the gut is very complex and is trained over time to break down certain types of food.
It's possible that without the intake of a certain type of food, the gut will slowly loose it's ability to digest it in the future.
Pro We don't fully understand how food nutrition works
The concept of "health" is inherently complex and not a fully understood field of science. Adverse effects of a poor diet could take a significant time to show symptoms and effect everyone differently.
This makes it very difficult to run tests on any new food intake system and conclude that it is a safe alternative to normal food.