I am no aero expert, but...
I have seen teeth like these used to increase local velocity, generate vortices, and help the flow stay attached to the surface (underside of the tunnel). Keeping the flow attached results in more downforce since more of the tunnel area is effectively used. If flow separates, turbulence and stagnation occurs, causing a loss of downforce and a possible increase in drag.
In addition, as Steve Lathrop said in post # 5 below, if the airflow is unstable and unpredictable, the handling will be similar, and possibly dangerous, or at least, unpleasant.
On my undertray (Citation F2000) I accomplish good, stable, airflow by using small wing sections at the end of the undertray to accelerate the air at the exit, keeping the air attached all the way to the end of the "winglets." Photos of bottom rear of undertray below.