I think the biggest myth is the aero wheel. I've seen guys swear that their $3000 Zipp wheelset makes them so much faster yet the data shows that the gain is very marginal. Zipp will show you some numbers where the biggest gains are at a high yaw angle yet the latest data is showing that we mostly encounter low yaw angles while cycling. The gains from such wheels under 5 degrees is pretty low.
With my minimalistic understanding of aerodynamics (won a CO2 car race in 2004 by creating the most lightweight and aero car ) I am clearly qualified to discuss on this topic
Now not being sarcastic, the aerodynamic design of an object is based on the air velocity and density it will experience. Generally aero wheels are designed to be effective and efficient at a certain air velocity. Which that velocity may never be experienced for some riders, and the gains of course may be a minimal percentage that will only be noticed on a larger scale.
That being said, the biggest aero and light weight mod we can all make is our bodies, lose some weight and work on a more aero posture. Aero wheels and frames are big "cool" factor objects (for non-pro riders), for some it is a reason to ride more. Which would not be a mistake