While vortex generators are not unicorns they aren't easy to use correctly. I'm currently simulating vortex generators on my car with CFD software. Placement and sizing are critical to get a positive effect from them. Placement and sizing are also dependent on several factors like boundary layer thickness and flow separation.
Its very easy to create more drag using them. Here is a link to a picture of an unsuccessful placement I did. (Note the two large detachment bubbles)
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