Some of the methods were summarized in all the previous threads. The people saying that Soylent were not being rigorous enough with the science were told that it's just someone doing a personal project and of course it doesn't need to be scientific.

It's quite clear the Soylent folks haven't. From their FAQ:

"Is it safe? We have been testing Soylent on ourselves for several months and the data shows it to have a positive nutritional benefit."

The well established, scientific method is a double-blind clinical trial, in which neither patient nor investigator know whether the patient is receiving the substance being tested or the control substance.