It's only rude if you believe that the customer has to respect the hard work and good intentions that went into the product, and make allowances if it doesn't "just work". They don't.
Similarly, I don't have to make allowances for people that can't manage basic civility. I wouldn't send back food with a "this is shit" comment at a restaurant. Even a fast food place. Because I'm interacting with a human being, and see no reason to be a jerk. As a side bonus, "sorry, this tastes bad, could I get something else?" has a higher likelihood of resolving the problem.