The most direct risk from frequent transfusions is
iron overload, a buildup of excess iron. Iron levels must be monitored and treated if necessary when patients get many transfusions.
The other risk is that transfusions are only a temporary solution to low blood counts and don't treat the underlying disease. You mentioned in another thread that your brother is 32 years old so supportive care alone is not a suitable long-term approach. The goal of his treatment should be to stop, reverse, or slow the progression of the disease so that he doesn't depend on blood transfusions.