Freeze dried dog foods can be very good for your dog. But of course, you have to review the ingredients of any freeze dried dog food to determine the nutritional value. Currently, there are some very nutritious brands of dehydrated dog foods—even some being grain free. (see: Recommendations)
It's hard to think of any negative aspects related to a freeze dried dog food, but if adding a bit of water and letting the food soak for a bit is a drawback, that would be it. Dehydrated dog food must be reconstituted before served, which makes it somewhat less convenient than simply scooping up a cup out of a bag of kibble.
Dehydrated dog food can offer excellent nutrition. Especially when the food is dehydrated in its raw state, the natural vitamins and enzymes remain intact. The process of freeze-drying the dog food keeps the food from spoiling. And, Because the food is dehydrated, it takes up less storage spaceprior to preparation.