Yes .... Full of contaminants after the blower still in the main supply. That air passes over that crap.A negative air system that taps into the main trunk line will remove a lot more soil and dbris that Ram Air will not. No argument there. The question then becomes, "Is it necessary to remove that additional soil?"
The philosophy with Ram Air and similar sytems - The air handler in the sytem is what would blow any soils out of the ducts and into the breathing air space of the home. Cleaning with Ram Air or similar systems uses the full pwoer of the systems air handler, plus air compressor, plus the vauum of the truckmount to loosen and remove soil. If the soil doesn't come out with the combined power of all 3 forces, it is not likely to come out with the air handler alone after youa re done cleaning.
So, Ram Air may leave some dirt behind. But if that soil stays in the duct work and never comes out, is it really doing any harm?
Maybe it comes out over extended period of time. Maybe there is enough soil and debris to restrict air flow. (Some construction guy left his lunch in the trunk line.) How clean do the ducts need to be? That is the difference in philosophy.