Not only limiting flow but above certain speeds, they simply can't move fast enough to properly meter the intake airflow. Reed flutter and resonance come into play followed shortly thereafter by reed destruction.

I always though of a reed valve engine as one that exhibits a better low and mid-range torque curve than a piston ported engine. A piston ported engine seems to breath much better in the high RPM ranges and if ported correctly and provided with a proper fuel, air and ignition source can easily rev to the point of mechanical destruction. A reed valve engine will just sort of level off in the high RPM ranges as the reeds become inefficient .... kind of like valve float in a 4 stroke engine where an engine only rev's so high and then it just stops turning any faster. Of course there are exceptions to all of these generalizations.