On the earlier Outlander the engine does about 3000rpm at 110 in parallel mode, similar to 4th in ICE cars and it spins the generator fast enough to still put some charge into the battery when driving on flat ground. Acceleration or going uphill sometimes needs help from the electric motors, so the state of charge varies a bit. My car will often put enough energy into the battery when driving at 110 to go for a couple of km without the engine running, it does that periodically on a long drive. It is a clever system and it seems some of the chinese models have also adopted this. Other PHEVs have opted for a different approach, like sandwiching an electric motor between the engine and gearbox and variations of that. The main disadvantage I can see in the Mitsubishi is the fact that the car can only start off using electric power and that can be problematic when starting with a heavy load on a steep incline. There is not much torque until the motor starts spinning! The other issue is the electric motors are always engaged and they will spin at quite high rpm at highway speeds, something like 12,000 rpm. It does not seem to be a real issue though as there are quite high mileage cars now and there are few reports of motor bearings failing.