This sounds like the best of both worlds when it comes to reliability. An overbuilt Toyota paired with the simple Tesla drive unit. One would think that with only four moving components it should be easy for the drive unit to last hundreds of thousands of kilometers. However, when the RAV4 EV was in development Tesla had only been a company for just over 5 years. This means that any issues Tesla had with the early Model S drive units would also be an issue with the RAV4 EV drive units. There were many issues…
First there was the motor bearings that failed creating excess side to side play in the motor rotor. This play caused the motor to emit a “milling” noise when moving. Another likely result of the play is the motor rotor coolant seal failing allowing coolant into the motor and inverter case. This coolant would not only damage the bearings further but also destroy the inverter if enough leaked into the case. Another problem is due to the use of an induction motor. With a permanent magnet motor the rotor is a permanent magnet and is spun by the magnetic field generated by the current running through the stator windings, this does not produce any excess current in the rotor(This style is used by other Tesla drive units and most other EV manufactures). However, with an induction motor the rotor is not a permanent magnet. The rotors magnetic field is generated by the current flowing thorough the stator inducing a magnetic field in the rotor. This generates excess current in the motor rotor. To prevent this current from damaging the motor bearings a silicon nitride bearing is used. This means that the next path to ground is the drive unit input shaft bearings. Tesla knew this when they were designing the motor and put some brushes on the motor shaft to ground out any current before it can get to the input shaft. This was not enough to fully remove all the current and the input shaft bearings are damaged as it makes its way to ground.
These issues affected a large majority of Model S and RAV4 EV drive units. Tesla revised the drive unit many times to fix the problem however when the drive units are replaced by Toyota under warranty they do not get the latest revision. This becomes a problem when the drive units fail in places that do not have a RAV4 EV certified Toyota dealership or the car is no longer covered under warranty.