What are some of the risks of a mastoidectomy?

Many possibilities. The risks include bleeding, scarring behind the ear or in the canal, perforation(hole) in eardrum, taste disturbances, dizziness, facial nerve injury, cerebrospinal fluid leak, meningitis, injury to sigmoid sinus(large vessel that drains brain), persistent hearing loss, and others. After all that said, a good ENT surgeon can take care of this with remarkable safety and good results.
Several. Changes in taste dizziness hearing loss infection that persists or keeps returning noises in the ear (tinnitus) weakness of the face.