That is a myth.

They may fly more accurately than your fixed blade heads at the moment because they are a shortcut for a untuned bow. A properly tuned bow with a matching properly tuned arrow and fixed blade heads are just as accurate as any mechanical head on the market.

If you wish to switch for a shortcut, your not doing yourself any favors. Your arrows are still not going to be perfect out of a untuned bow. The proper thing would be tune your bow, tune your arrows and you may not want to switch.