It’s not often that a CI will fail, however, they are electronic devices and all the companies have run into trouble from time to time. The failure statistics are supposed to be updated on a regular basis by the CI companies. Thankfully they are all highly reliable and, for the most part, have close to 100% reliability statistics.
Replacing a failed CI requires a second surgery. It’s usually much faster (no drilling needed if the exact implant is used) and the recovery time is faster. Activation is also usually faster. It’s really not something a person getting a CI needs to worry about. It can happen, but the chances are high your CI will last for a very long time. If you have a replacement device, you might have to do some rehabilitation again, but it’s usually a much faster turnaround for things to get back to where they were, or better.