Asking what constitutes a "good" or a "bad" diet for dogs, cats, and other pets is a topic which people often have strong opinions about. This makes it difficult to reply objectively to questions like this.
I think most people accept that the manufacturers market their products to people ... not the animals, obviously ... by resorting to emotional appeals rather than verified health benefits.
One has to dig into the ingredients listed on the package to make a more informed choice. Even then the discussion is seldom definitive. There are many ingredients best described as "controversial". By this I mean many people have strong, anecdotal feelings that an ingredient is bad in some way. But there are no definitive scientific studies which prove that its use leads to health problems.
Of course, lack of proof is also not proof that an ingredient is benign. It merely means we don't know. Hence the frequently expressed strong opinions.
One source I have referred to is the Dog Food Advisor site. What I like about the site is that the commentary discusses each ingredient listed on the package. They offer brief, understandable explanations for cryptic industry terms such as
brewers rice or
chicken by-product meal.
Here is a link to their comments on Royal Canin Medium (Dry). If you refer to it, you should cross check the ingredients on the food you are feeding your puppy to be sure they are talking about the product you are using. They do not rate each product in a manufacturer's line. Instead, they select one product in the line to represent the others on the assumption that they will be essentially identical.
I could not be sure which Royal Canin product your vet was talking about from the information you provided in your question. I noticed that Dog Food Advisor does not discuss veterinary food products.
My personal opinion is that Royal Canin does not impress me as dog food goes. But it is "better" in terms of the quality of the ingredients than a lot of other lower cost, foods. Dog Food Advisor ranks it as 2.5 out of 5 or (slightly below) average by their criteria.
I am also not sure what benefits the Royal Canin sold by your vet would offer. Do you have a specific product name? I personally would feel a lot better about a product recommendation if I knew that the vet would not benefit financially from it. But who knows.