Vet Tech here. Current research says a number of things: 1) it is obscenely difficult to balance nutrients appropriately in a raw food diet, leading to significant health problems if done poorly. 2) There is no benefit that has ever been found in any research. 3) Raw-food patients are ALWAYS considered immunocompromised when they are in the hospital, and are also always considered vectors for disease that they otherwise might not be.

The takeaway is that, while it may be "okay" to give to dogs, there's no good reason to do so, and a lot of reason not to do so

It's a lot more complicated than carnivore/omnivore/herbivore. Many herbivores will eat meat when the opportunity presents itself, because it is a very nutrient-dense source; likewise, carnivores will graze and eat many different types of vegetables to supplement their diets.

Obligate carnivores get the overwhelming majority of their nutrients from meat that they hunt; that meat gets the majority of its nutrients from plants. The variety of plants they eat will affect the nutrients available to their predator. So, one of the issues with raw food diets is that most of our food animals tend to be fed shit food, meant to bulk them up, rather than nutritionally balanced ones; that nutrient loss is absolutely felt in raw food diets.

Dogs, though, are not obligate carnivores. They can survive as omnivores or scavengers, though the latter, of course, is the worst option for them.

But... I guess the long and short answer to your question is "it isn't their wild diet," and also "animals die younger in the wild, eating their natural diet"

You're right about the dog poop size! Higher quality dog foods will tend to create smaller poops. But, I will say that you don't actually just crap out corn - they have a cellulose shell that we don't digest, but most of the meat of the corn does get digested

The current understanding in the field is that there is "no benefit," which means that you're going to be wasting your money for no reason. Currently, it is not clear whether or not there are any increased risks that come from grain-free, specifically; but, nutrition is complicated, and it takes a long time to figure out whether or not something is good.

Most of the time, when an owner says they feed their pet "grain free" diet, we roll our eyes, but move on and don't argue, because there also isn't substantial research to say that it's bad.

Vet Tech here. I've literally never once met a Clinician who has been paid any amount of money by any of the Big 3 companies to sell their food. We use RC, Hills, and Purina because they are companies that have done longitudinal research on their diets, and how those diets affect the animals that feed them. We don't recommend the shitty foods by those companies - Beneful by Purina is one we call trash, for instance - but their good foods have been shown to be the best available.