Aluminium and plastic are all about cost saving and weight reduction. Alloy is adequate for heat transfer but is near impossible to repair in the event it fails (and it will almost always fail where the cooling tubes are sealed to an end tank) The thing that bugs me about these type of radiator is the plastic tanks cracking. They are nowhere near as resilient to critical heat cycles if there is a leak somewhere else in the system and coolant levels drop. It's like it sends it on a death march once something goes wrong.