Yes, the biggest drawback is that soy (in the form of soy meal) is the best plant-based source of inexpensive low fat methionine, which is the most important of the critical limiting amino acids, particularly early in life. So important, in fact, that a limited amount of synthetic Methionine (dl-Methionine n the feed label) can be added to chicken feeds, even those carrying the "organic" label. It is, in fact, impossible to fomulate an all plant chicken feed that meets the optimum recommended Met levels for adult layers, much lsss hatchling dual purpose or "meaties" without using dl-Met and Soy Meal or one of few (all much more expensive) substitutes. If not trying to formulate a synthetic-free mix, but still soy free, flax seed is a common (and expensive) substitute which brings up the total fat content and adds significantly to the price. If willing to skip "vegan" (and you should), then porcine blood meal, fish meal, shrimp meal, crab meal, menhaden (fish) meal are all ways to spike the total protein levels and simultaneously raise Met levels closer to accepted targets.
