I don't have a lot of insight into the science behind it, but one thing that always makes my partner (PhD in Epidemiolgy) slightly angry is that while Soylent does tick all of the "amount and type" boxes for what people consider to be part of a healthy nutrition, the real long term effects are basically untested.

Even in "naturally grown regular foods", the field is still pretty young and we can barely make good recommendations for those. They just happen to be what we eat and as long as they're not crazy processed, we have a few thousand years of experience with them not necessarily killing us immediately.

Areas of research like Metabolomics or Proteomics (https://en.wikipedia.org/wiki/Omics) are still in their infancy, so while Soylent does mix a bunch of ingredients that all fall into the right areas, the long term effects are basically unknown. And that doesn't even take personalized medicine into consideration where your gene markup might actually have a lot to do with what might or might not be good for your personally.

Again, I don't have any insight besides some armchair quarterback knowledge about nutrition, but it seems like most people that do don't think it's a particularly great idea to eat it more than 'every now and then'.

That being said, when compared to twinkies and sugary sodas it might still be better for you. Having it as part of a daily diet though might not be a good idea.