Name one that a) provides a proper amount of needed nutrients and b) is as simple and convenient as Soylent (or equivalents).
And purple corn meal for carbs: https://www.amazon.com/Z%C3%B3calo-Peru-Organic-Purple-Flour...
Organic virgin coconut oil (in glass jar) for fats: https://www.pureformulas.com/nutiva-organic-extravirgin-coco...
Soylent is $2.69 / 400 cal and vegan.
To get the same macro ratios you don't need the corn meal, the shakes have plenty of carbs, you can do 1 shake @ $3.33 + 15g coconut oil @ $0.55 + a few cents for the multivitamin = ~$4.00 / 400 cal.
(You may still want the cornmeal for fibers and slower carbs, since the carbs in the shake are all from milk and cane sugar.)
So it's more expensive, requires preparation, is not vegan, and the glycemic load is probably worse. Beautiful.
Cow milk has actual estrogen. Because cows are mammals. With actual sex hormones. Which binds strongly to estrogen receptors, because that's what they're for.
I know which one I'm going with.
To be honest I don't think it's that big of a deal in either case (soy or dairy), for occasional consumption, I eat both on occasion. But if we are talking about replacing meals with a dairy shake, or GOMAD (gallon of milk per day, a 'traditional' recipe for fast weight gain), I might be a little bit concerned.