if you do not have a sealed system, meaning the radiator is always full the crossflow will still function when the coolant level is low. If the coolant level is low with vertical tubes you will be sucking air, lots of bubbles, poor heat exchange internally in the engine, and overheating.
However, with either, if you keep it properly serviced there is no problem.
Off-roaders prefer the crossflow because sometimes we run a fan into the radiator and we want to creep out of the backcountry rather than walk out.
