In theory the reeds seem to be better on twisty tracks and the rotary valve versions when run at top performance, which means up to 21,000 rpms on direct drive configuration, are very fast on long straights, though that means replacing a piston basically like at least every day you run it. On dirt it is a different story. They made different valves for different tracks, and some engines were ready to take bigger carbs for the old FIA/CIK Super A class while some just had an intake for a smaller 24mm carb, run in the Formula A class. To run in this class in Europe it was required to have experience and even have won at least some National Championship in many cases, which means that many of these engines were top factory rides were prepared by the best builders, like Faluga in Spain, for example. When FIA/CIK moved to water cooled engines the evolution of rotary valves basically froze, and reeds kept improving and actually became faster and more powerful than previous rotaries. In fact seems like many of the rotary valve designs by rotax were copied by other brands. Certain particular Rotax models were weaker than other contemporary engines, as they came from the factory, but I heard these issues were solved by changing rods and pistons.
