Well designed PHEVs can actually be simpler than pure ICs (at least on the hardware side. To build a combustion only car well, you need to balance efficiency, power, and responsiveness. This means you need all sorts of complicated tech, like correctly sized turbos, variable valve lift, variable valve duration, etc. In a PHEV, otoh, you have an electric engine (which can also steal power from the driveshaft), which means you don't need to worry about responsiveness of the combustion engine. You can fill half a second of turbo lag with the motor, and optimize for narrower RPM ranges since you can charge/discharge the battery to keep the engine running in its happy place. You also no longer need fancy and complicated brakes because you can do 99% of your braking with regen.

All of this does come with more complex software, but the hardware can end up with significant simplification.