What's the risk of hearing loss and or facial paralysis after mastoid surgery?

Mastoid surgery. Mastoid surgery itself may not affect hearing. But there is a small risk of facial paralysis, a complication.
Both are infrequent. Chance of severe sensorineural hearing loss is approximately 1 in 1000 mastoid operations. Chance of facial nerve injury is approximately 1 in 10, 000 ear operations.
Small risks. Making hearing worse, or facial paralysis are known risks of mastoid surgery, but are small risks. Surgeon's experience and training are key.

Related Questions

Cyst in my middle ear that has caused hearing loss, facial paralysis and vertigo. Is it better to have surgery to remove it or put up with it?

Cholesteatoma. You should have surgery asap as the lesion will destroy your hearing and can erode all the way to your brain!
Depends on cyst type. It depends on the cyst type and if it is associated with a a tumor. There are certain tumor cysts that "grow" and cause pressure on the surrounding structures that can lead to the neurological problems you describe. If there is a tumor associated, it is best to treat that surgically as it will keep growingi. Also if you have serial scans that show growth of the cyst, it should be removed.