How serious is hemophilia?  

Very serious. Interferes with blood clotting, so subject can not easily stop bleeding when it occurs. Not just open bleeding, but into joints which when started will only stop when the pressure within the joint equals the blood pressure, so there is severe joint swelling followed by fibrosis (scarring) within the joint. Similarly uncontolled bleeding can occur into the GI tract and/or lungs.
Hemophilia. Hemophilias vary in severity. For a better answer, i would need to know what specific type of hemophilia you are curious about and what level of the affected clotting factor the patient has. For instance, patients with severe hemophilia a (factor viii deficiency) bleed with minimal trauma and need frequent infusions of preventive medicine. Others can go a lifetime with only minimal problems.