Costco has a strong record of getting me to buy food products that are appealing to me for a while, but that I now consider bad for my health.
About the only food products I will still eat that I can buy at Costco are boneless poultry white meat, Kerrygold butter, olive oil and fresh asparagus.
Costco stocks many fruits and vegetables, but most of them are much too high in sugar to be good for me, I now realize. (Berries are on the low side in sugar content, but Costco does not stock frozen raspberries or blackberries, and I avoid fresh berries because they're about 50 times as moldy as frozen ones.)
Examples of foods that I eat a lot of that I cannot get at Costco are cabbage, radish and (frozen) tart cherries.
In contrast, Whole Foods carries most of the 3 dozen or so foods that are good for me to eat according to my current understanding of nutrition (but not the tart cherries). I would probably be significantly healthier if many years ago I'd never become a Costco member and stuck to Whole Foods and Trader Joe's.
There was a great comment on Reddit from someone who worked in the meat department that highlighted this comparison with specific examples but alas I am unable to find it.