MgO: good for making furniture for a 1400°C furnace, not so good as a source of magnesium.
https://faq.soylent.com/hc/en-us/articles/203962069-Vitamin-...
https://faq.soylent.com/hc/en-us/articles/204547005-Magnesiu...
I'm not enough of a nutritionist to say whether that's really sufficient, but they do consult with nutritionists, so it seems likely they've at least thought about all this stuff. Proving there's something wrong with Soylent is going to take more research than anyone's likely to casually do for an Internet discussion.
Similar for the Magnesium, it was chosen for a neutral taste rather than based on bio-availability. Even though Magnesium Oxide is barely processed by the human body at all.
I'm not going to get into it because this stuff gets complicated really fast [1] and two non-scientists debating it on the Internet gets silly.
[1] https://ods.od.nih.gov/factsheets/Magnesium-HealthProfession...
Vitamin D2 instead of D3
magnesium oxide instead of one of the several well absorbed forms like magnesium citrate
calcium carbonate instead of calcium citrate
zinc sulfate instead of zinc picolinate, zinc gluconate, or zinc citrate
chromium chloride instead of chromium picolinate
selenium as sodium selenite instead of selenomethionine
In particular, magnesium oxide has very low bioavailability, and I would suspect that if Soylent is your only source of magnesium, you could potentially become deficient to some degree.
There are other ingredients that are not optimal, but those are perhaps less serious.