Yes a study is not just a study. If I own a pet food company. I pay xyz company to do research on the best diet for a puppy. Guess what their study will state that my kibble is the best. It meets and exceeds that standardize guidelines for a puppies health.
I have often stated "follow the money." Studies are often biased towards the ones whom are paying for the study. Therefore, most studies should not be trusted. A study done by a non biased party I more reliable.
Unfortunately, there aren't many unbiased people doing study using scientific methods.
Not a big fan of grain free food to be honest. Especially for the dogs. Seems to be a better option for the cats though https://www.catfoodpoint.com/best-grain-free-cat-food/. Anyway i think it is always a good idea to discuss food changes with your vet and check closely how pets react on that.
@eliteguardianpresa stated "Unfortunately, there aren't many unbiased people doing study using scientific methods.
In this one simple sentence is so much truth that most will not understand it. Companies mostly fund research with a given hypothesis to prove and the abstract of the research normally tells what the outcome will be. Following the money and who tends to benefit from the outcome will give a good idea of what the research promote.