Hoichi. Hi!!!! I've been to CI evaluation. Lemme tell ya......The doctors are required, by law, to inform you of any and all side effects, no matter how rare. They also try to set the bar for expectations low. If someone has been deaf for most of their life they might even have to have a physiological exam to make sure the patient doesn't have too high expectations.
They used to have higher fail rates because, well now they have better technology and procedures, but also because they were implanting people that shouldn't have been implanted, people that weren't really good candidates. Older children and adults that are prelingually deaf are not good candidates. People that have never had effective hearing aid usage are not good candidates. Current diagnostics are better at checking the viability of the auditory nerve. They implanted people that their nerve had atrophied.