How risky is hemangioma in humans?

Depends on location. Some hemangiomas are not serious at all. Others because of their location can be a serious problem.
Very few for most. Hemangiomas occur in infants and appear during the first few weeks of life. For those that remain small or flat, simple observation is fine. For those on the head and neck, or those that grow aggressively, these need to be seen early and often benefit from early intervention. Those that occur around the eye, lips, nose or airway can cause rare but life threatening conditions.