How useful is surgery in treating brainstem cancers?

Not usually. Surgery is usually not indicated for brainstem cancers.
Surgery is risky. Brainstem cancers are usually intrinsic to the brainstem. That is, they invade the brainstem and are infiltrative. Surgery to remove them is usually very risky because of damage to the surrounding tissue. Furthermore, it unlikely to be successful because the brainstem cancers have "fingers" that cannot be removed.