All pinball machines are subject to technical/mechanical failures. For the 3 NIB stern's I have bought, I have had the following issues:

1) Spiderman - Insert Lamp not connected. About 20minutes of tracing the lamp matrix/wiring harness and 2 minutes to 'repair'.
2) X-men - Auxillary board failure. 5 minutes to replace board once it arrived. Magneto Magnet noise, 5 minutes to install an additional e-clip. Wolverine Target not registering correctly - 20 minutes to disassemble, fix, reassemble.
3) Star Trek - Warp Ramp not smooth. 5 minutes to adjust and follow the pinside recommendation.

That said, I've had a much worse time on on B/WMS machines. They are older, and things on the board start to go. Those are more challenging repairs than the above issues.

So. YMMV, but, be prepared to at least have a small set of tools or a good responsive tech.

-Wes