I've had a Neptune MAH7500 for about 20 years. It's a work horse. In retrospect I wisely got the extended warranty, and sure enough before that was out, it needed some major repair. As I recall, the big drive wheel at the rear had cracked. Replaced free of charge. Has been trouble free since then, although it's possible the belt needs replacing and/or the motor moved to tighten things up a bit.
If I'm not mistaken, the 7500 doesn't have wax motors; it uses solenoids instead, and those have never failed. They are a bit loud, though. Of course I might be wrong, and I'm too lazy to go check the manual about it.