I worked at a place that had the contract to test &/or repair Cochlear devices.
At that time, the processor was a separate unit from the "behind the ear" microphone, although it was already a very compact unit.
We had very few major problems, even in those early days.
Most of it was replacing the microphone ( from memory, you could open up the "behind the ear device" & do it at "component level", in much the same way as we repaired conventional hearing aids).
The external coil was replaced as a complete unit if it failed.
Obviously, we had nothing to do with the implanted bits, but they were extremely reliable.