I fixed them for several years as a summer job along with other amusement equipment. An arcade with about 150 slots kept me busy during the peak summer periods when the machines were worked hard (65-70 hours a week worth of busy--loved the OT!). I have an electromechanical Gottlieb machine--a Sing Along, for the curious--made in the mid 1970's, and other that rubbers and light bulbs I have never had to fix anything since it came out of the arcade where I worked despite several moves and (for several years) pretty heavy use during parties and such. Even the coin mechs still work fine.
Pinball machines were made only for a few weeks per model back then, and expected to last a few seasons in arcade use without too much repair. For home use, they ought to be pretty bulletproof, but YMMV.
John