Its not unhealthy if prepared correctly and omega 3 fatty acids, which are VERY healthy, are obtained from some types of fish. Its also a very low fat food and, if eaten as sashimi (no rice) its also low carb.
As it's made of raw fish, rice, algae and such things, it has valuable vitamins and other components. Of course, the ingredients have to be fresh and properly prepared. It's certainly better than a burger.
The main ingredients of traditional Japanese sushi, raw fish and rice, are naturally low in fat, high in protein, carbohydrates, vitamins, and minerals. Other vegetables wrapped within the sushi also offer various vitamins and minerals. Many of the seafood ingredients also contain omega-3 fatty acids, which have a variety of health benefits.