Captin,

The downside of reed valves to me hinges on maintenance issues. Reeds can break, or go soft and leak, or stick shut when the engine is stored for a long time, etc. When I was a Go-Kart racer I ran into all the above at one time or another.

Rotary disk valves don't have these issues, give more precise timing and allow higher engine RPM. However, disk valves add complexity, weight and cost. Tradeoffs.....

Before someone tries to start a war, I'm aware that there are plenty of reed valve engines out there and I'm not going to argue about it with the reed valve proponents. ;^)

Cheers,

CR