I think a flash is superior.
I also think a well tuned complex piggyback is perfectly adequate.
A flash just allows you to control much more, a short list: lift, duration, limitless timing control, the way adaptation works, how much knock sensor noise to retard timing, boost/throttle behavior on hard shifts, individual gear based torque limits, redline, throttle closure behavior related to boost control, limitless control of boost at any rpm without limps, and more.
Though, I feel like the piggys we have today are safe and adequate, and control more than one or two things.
'08 Black Saphire/Black Z4 M Coupe
RIP Gretta: Blue Water/Lemon 135i. Died to save me.