It's a fair assumption that adding extra systems to the car makes it overall less reliable, but it's not necessarily true. Electric motors, compared to IC engines, are extremely simple and reliable. The servicing guidelines for the electric drivetrain in my hybrid is essentially "replace the battery if it stops holding enough charge", there is no schedule for any routine maintenance of those components. Adding the hybrid system also reduces the wear and tear on the conventional drivetrain and brakes. Hybrids can do regenerative braking, which means that (for my vehicle at least) most of the braking down to maybe 10mph is done by regen, which functionally has no wear and tear. The electric motors also assist the ICE at the times where peak wear and stress occur, reducing the load and stress on the motor, and extending it's lifespan. By adding the hybrid system, the overall reliability and lifespan of the vehicle is increased rather than decreased.