The Model S drivetrain is reasonably reliable. Most of the reliability problems (and I believe they have an average score now, not great but not terrible) are other parts of the car. Tesla likes their unnecessary "cool" tech, which also tends to break. The Model S has the extending door handles, and the X has ridiculous doors. Not that it's limited to these aspects of the vehicle, of course.

Note that the Prius doesn't have a CVT the way you'd normally think of one. It has a fixed planetary gear set which is able to somewhat emulate a CVT by using two electric motor-generators in conjunction with the engine. I mention this only because "CVT" usually makes people think of painful friction-based transmissions, whereas the Prius's transmission is much simpler than even a normal transmission. Your point that engineering skill and manufacturing quality are the main factors is spot-on.