ARM boards with slotted RAM use the same type as x86 (although mostly LPDDR, as found in laptops), so I assume there isn’t any difference that is related to the CPU architecture.

Except the circumstances in which they run on gasoline are not that exceptional: https://theicct.org/publication/real-world-phev-use-jun22/ And the lower weight of the battery is partially offset by needing an additional drive train, a transmission with multiple gears (purely electric cars usually have fixed transmission) and other smaller parts required for the combustion engine. These also increase the required maintenance a lot (e.g. requiring oil changes).