Feeding one or the other exclusively would be bad, yes, but as DallyTsuka said, neither is bad as part of a varied diet. The thing about seed is that it's relatively high in fat and low in nutrition; it lacks vital nutrients, such as vitamin A and calcium. A seed-only diet will lead to nutritional deficiencies, fatty liver disease, obesity, etc. Pellets are important for birds that will not eat enough fresh vegetables and greens, but since there's still a lot we don't quite understand about parrot nutrition -- and most pellets are not formulated for specific species -- an all-pellet diet can also be potentially dangerous.
As for Zupreem fruit blend itself being bad, I would not feed it to birds who have weight issues, because of the added sugar. But if it's the only pellet your bird will eat, and it's not ALL they're eating, I don't think it's terrible. I do feed it to my tiels. I've taste-tested many pellets and it does have the most flavor of any brand I've tried so I understand why it's a favorite. Harrison's tastes like cardboard and sawdust, unfortunately, and most other brands aren't much better. I know Zupreem changed their forumula for the fruit pellets last year to contain fewer artificial flavors and more natural ones, so that's something.
A bird can get by without eating pellets just fine, if it eats a variety of vegetables and fruit, grains, legumes, nuts, seed (for seed-eating species), sprouts, etc. But I personally feel better having them around as a source of vitamins (especially vitamins A and D), even if it has to be Zupreem fruity.
