One of the cost cutting decisions when designing the Model M was using plastic rivets. Some of these rivets break and the keys stop responding. My 1997 keyboard still have all the rivets intact, so I suppose that it might have to do with the treatment that the keyboard receives. This video shows how to replace the rivets with bolts: https://youtu.be/dStgzMpOTOA

Sometimes the circuit board might fail too. You can find replacement boards for them too. My keyboard is a 4th generation, which is almost identical to the design sold by Unicomp today. The circuit board died last year and another Model M user designed a compatible circuit board that adds full programability with the QMK keyboard firmware, so I will install that one instead. In addition to programability (Win/Command key, international characters, emojis), the new board converts the keyboard to USB. So yeah, this circuit board really modernizes the keyboard.

https://beta.docs.qmk.fm/

I've not had to do anything other than light cleaning. When I moved a few years ago all of my keyboards were in the same box. Apparently it got really hot because a smaller (TKL) style Lexmark era IBM keyboard in that box melted, but my 2 Model Ms were fine.

These things are built like tanks. I don't think I'll ever have to do any repairs on them.