The problem I have with statements like this is that it gives no context for what 'not healthy' actually means (and, by extension, what a 'perfectly healthy' diet is).

For example, I often don't hit 3 of the 5 recommended fruit/veg in a day and I consume far too many fatty foods. Over an extended time frame this will probably cause me to become overweight and deficient in vitamins. Given this, it'd be useful to know what the advantages and disadvantages of soylent would be in my current situation.

Do you have a link to the studies? They presumably give more information about their benchmarking.