Legumes and soy in particular is a pretty common allergy... it's nearly impossible to get sufficient protein without meat if you have a legume allergy.

The impact of non-natural feeds on the overall nutrition profile for chickens and pork are larger than with ruminant animals. Chickens have been bred and changed a lot through environmental manipulation to grow much faster than in nature.

There are a few breeds of cows that are producing more muscle mass than most, they've gotten quite a bit larger through breeding as well, though the difference in time to maturation doesn't come close to what we've done with chickens... I'm not sure it's for the better though.