Ok. We have electric motors coupled to an internal combustion engine. Suffice to say that on the EV side the electric motors are 100% reliable. In the last 6 years reconditioning and repairing the battery I have not come across 1 instance of EV drive motor failure. Putting this aside we will look at how the internal combustion engine and transmission contributes to this reliability.
Maximum efficiency of a conventional engine occurs at around half of the engine's peak power output. So if the engine has it's peak power at 12,000 RPM maximum efficiency would be around 8000 RPM. This means that at the low RPM end ( low speeds and idling) and the high RPM end it is not optimum.
Toyota uses the Atkinson cycle engine. Maximum RPM 4500 in the Generation II Prius (2000 - 2003) and 5000 r.p.m in the Generation III Prius (2003 - 2009 ). The lower end of the spin rate 0 - 2500 is taken up by the EV drive, leaving the ICE to work in it's most efficient range of 2500- 4500 RPM.
Reducing the spin rate also allows lightweight parts to be used, therefore reducing inertia and friction losses.
The crankshaft is offset from the cylinder axes so that during the combustion stroke the force from the piston is transmitted to the crankshaft through a straight rather than tilted connecting rod.
The valves have narrow stems and low force springs to reduce energy lost in operating the valves.
There is no starter motor. This function is taken up by the electric motor MG1
There is no alternator. This is taken up by a DC/DC converter. As there is no moving parts in a converter it is more reliable.
There are no belt driven generators. The same EV motor that drives the vehicle also acts as a generator.
The planetary gear system used does not employ a clutch and reduction gears. A chain drive transfers power efficiently avoiding losses through axial thrust.
No clutch means we are rid of another gadget that requires frequent replacement and poorer efficiency as they wear out.
Simplicity of the design in the transmission devoid of reduction gears makes for a transmission system that I have yet to see fail.
Less is More
The sum of it is that in the Toyota Hybrid design, the system is simpler and less complicated. Coupling an electric motor to the system sounds complicated but in reality they have put in a more reliable drive train to take care of the low end spin and employed an ideal ICE design for the top end spin. This marrying of the best of both world contributes to the efficiency and reliability of the Toyota Hybrid engine.