In my experience, some people have ears that love hearing aids and others have ears that hate them. For the former group, their hearing aids keep on trucking for years and for the latter the hearing aids go in for repair frequently for debris or moisture issues. I don’t see any strong correlation with which manufacturer they wear.
Yes, wax can push through the filter and into the receiver if the wax guard is not changed regularly enough. Equally likely is debris in the microphones from sticky fingers. These are the two most frequent reasons for breakdown that I see (we have a microscope in our clinic which is probably uncommon, and so these problems are often invisible unless the manufacturer sends a photo). After that, probably damaged wires if someone has insertion difficulty for RICs, or batteries shoved into the body of the hearing aid for customs. Then moisture issues killing the microphones or the amplifiers (for my region, this probably ranks higher for more humid places). Then dogs. Or drops, for custom aids which can be less robust than BTEs in that situation.
Recently had a hearing aid come in that spent the winter outside under the snow (on the driveway, being driven over by the car). It was a bit scratched up and there was some corrosion in the battery compartment, but after I cleaned it out it worked perfectly. (+1 for Unitron.)