I don't remember who it was, but someone tested x amount of canned pet foods. VERY few had the appropriate amount of vitamins and such. If I remember correctly, they were lacking, rather then having too much.
I'm glad that one of the first things someone taught me (when I fed canned) was to rotate, rotate, rotate. Rotating saved our cats from a baaaaaad situation. Even with feeding raw, I still rotate brands of meat and proteins.