It gunks up mass air flow sensors, most measure the air by heating a portion of it and then sensing what it heated to. The oil coats the heating filament and the sensor, disabling them.
I'm working on an oil separator for an aftermarket filter system, basically what I have come up with is the tube from the engine comes into a can near the bottom at an angle to spin the air and drop out some of the oil mist, the drain to the crankcase drops out the bottom along the edge, and the air to the intake comes out the top center. Pretty basic, but it'll cost me $5 to build versus $60+ for a "race" oil separator.