I Totally agree with you. What may be good for one cat, may not be good for another. I had been going completley nuts-and almost broke,running around searching for the perfect food for my cats. One particualr brand, which I heard very good things about, turned out not to be what my cat Sammy needed, because his fur turned dull, and coarse.Since I switched, his fur seems to be changing for the better.
Also, dairy products don't agree with many cats systems, but for some they do fine with it. My little Romeo LOVES yogurt, any flavor,cheese, milk, and eggs.And he has never had any stomach problems yet. Sammy on the other hand, drank some milk from Romeo's dish, and he got gas so bad, I thought his backside was going to explode!! He truly brought tears to our eyes that evening!
I considered trying a raw meat snack as a treat for my cats,but I changed my mind, because I am concerned with samonella, etc.
If cats coats are looking shiny, they are healthy, and enjoy what they are eating, then I agree to stick with what you find to work best for them.