Because of the relative low load and long hydraulic retention time constructed wetlands turn out to be the most reliable wastewater treatment systems. Hydraulic overloading tends to be no problem, this effectively happens every time whenever it rains heavily and of course any well designed structure can cope with heavy rainfall. Organic overloading is also possible (when temporary many more individuals visit the locality or location) and more pollutants will be simply buffered in the constructed wetland itself. After the peak the bacteria in the wetland will simply take some time to process all the pollutants before going back to the steady state of before.
Only in case a constructed wetland is overloaded for a prolonged period of time, the quality of the effluent (treated wastewater) will worsen. In that case one can argue that the constructed wetland was designed too small.