Belgian Shepherd dogs tend to be very healthy and robust in general. There are some inherited disorders that a would-be Belgian Shepherd dog owner will want to research. Hip dysplasia is uncommon but may be on the rise. Eye disease is somewhat more common. Your breeder should be able to provide documentation that their breeding stock has no apparent inherited defects. Incidence of epilepsy is difficult to determine. It seems to vary according to variety and lines, although it is doubtful that any lines have had zero incidence. Anesthesia sensitivity is a real concern for any Belgian Shepherd dog, as Belgian Shepherd dogs tend to carry little body fat, and can be overdosed, particularly with certain drugs.
Below are links to pages concerning these issues and others that can affect any dog, regardless of breed.