Biology is hard. Humans are really, really variable. Look through the comments on this page - you'll find people who couldn't keep down one bottle, or who love the bottles but can't stand the powder, or who have had zero issues, or who find the entire idea disgusting.

And these reactions may also change over time. Medical science can only really draw firm results from large-scale tests involving highly-controlled groups - and even if Soylent were able to run those kinds of tests and optimized for the most universal solution, the end result would probably still disagree with someone.

Plus Soylent has been presented as a replacement for all other food, and some people use it as such. I would think that would make it more likely that their users would experience problems, since it would mean greater and constant exposure to ingredients that might not cause issues if consumed less frequently.

There isn't really any bright line between edible and poisonous. E.g. if you buy a bunch of field guides, the exact same plants and mushrooms will be listed as edible in some books and poisonous in others. There are lots of species that are widely considered poisonous, but many people eat them anyway without any ill effects. And others that are widely considered edible that people get very sick from eating.

Some of the best advice I got from a well-known mycologist is when in doubt about whether something is edible, set up a TV in the bathroom first.

"Every mushroom is edible, but some only once." -- Czech proverb (https://en.wikipedia.org/wiki/Mushroom_hunting#Safety_issues)

Are you serious? Any mycologist or survivalist worth their salt will tell you to avoid any mushrooms you could not identify with 100% certainty because there is no regret if you had them. Some manuals may advocate (in very desperate situations) eating a small piece of an unknown species to see if you have a reaction before consuming more, but that is still pretty dodgy since not all toxins have a short incubation period.

On the other hand, generally recognized as edible: many people consume it regularly, most doctors wouldn't tell you to never consume it, recipe books include recipes using it, etc.

Many (identified!) mushrooms are on a similar level of toxicity: one is fine, a kilo will make you nauseous, twenty kilos can kill you. Whether this is toxic or edible is to some extent a cultural construct. The dose makes the poison.