I used to feed Vitalin muesli type food to a previous dog and he did reasonably well on it, though to be honest I'm not sure he actually loved the food (!) but I think there are differences between these and the extruded foods and it's down to digestibility - muesli foods tend to be wheat based and wheat is not as digestible as cereals such as rice or maize, and I believe that the cooking and extruding process also makes the carbohydrate and protein more digestible.
As a result you generally have to feed more muesli food and get more poo out the other end!