People live off of Ensure, Boost, Nutren, et al., as their sole source of calories for years on end and don't seem to develop nutritional deficiencies. Maybe Soylent is more "natural" (whatever that means), but the established alternatives (which, in addition to being sold by people that know what they are doing, are cheaper) have a proven track record. At best, Soylent meets basic requirements, but frankly I have my doubts, given how they (last time I checked) STILL didn't have an actual nutritionist checking on the micronutrients.
(Nutren, which although drinkable and flavored, is specifically advertised as being feeding-tube compatible... people have lived for decades off the stuff.)
And, as you point out, the Soylent people keep making major changes to the formula, demonstrating that they still don't really have any idea what they are doing. They should have started with an established nutritional drink and worked to develop an "open source" equivalent. Starting from scratch out of utter ignorance was the exact wrong approach.