My view, that constructed wetlands (CW) are not an appropriate wastewater treatment system, applies principally to surface-flow CW and horizontal-flow subsurface CW, particularly their use in temperate climates.

 
1.  CW are (almost always) used as secondary or tertiary treatment processes, so they require pre-treatment (in, for example, a septic tank if they are secondary CW).  While this may appear obvious, not everyone seems to realise (or openly admit) this.

2.  There are serious questions on the role of the plants in CW − see, for example:

C. C. Tanner (2001).  Plants as ecosystem engineers in subsurface-flow treatment wetlands.  Water Science and Technology 44 (11–12), 9–17.

J. D. C. Baptista, T. Donnelly, D. Rayne and R. J. Davenport (2003).  Microbial mechanisms of carbon removal in subsurface flow wetlands.  Water Science and Technology 48 (5), 127−134. 

T. Manios, E. I. Stentiford and P. Millner (2003). The removal of chemical oxygen demand from primary-treated wastewater in subsurface-flow reed beds using different substrates. Water Environment Research 75 (4), 336−341.

D. D. Mara (2004).  To plant or not to plant? Questions on the role of plants in constructed wetlands. Paper presented at the joint session of the Ninth IWA International Conference on Constructed Wetlands and the Sixth IWA International Conference on Waste Stabilization Ponds, Avignon, France, 30 September.

3.  CW (at least horizontal-flow subsurface CW) can remove nitrogen by nitrification-denitrification (see Tanner’s paper above), but they generally do this well only in summer and in fact achieve little or no removal in winter − see:

D. D. Mara and M. L. Johnson (2007). Ammonia removal from facultative pond effluents in a constructed wetland and an aerated rock filter: performance comparison in winter and summer. Water Environment Research 79 (5), 567−570.

and they require more land than, and (at least in the UK) cost more than, secondary facultative waste stabilization ponds + rock filters − see:

D. D. Mara (2006).  Constructed wetlands and waste stabilization ponds for small rural communities in the United Kingdom: a comparison of land area requirements, performance and costs. Environmental Technology 27 (7), 753−757. 

4.  For small communities of a few hundred people primary facultative ponds, or septic tanks and secondary facultative ponds, plus a rock filter (aerated if ammonia removal is required) are more appropriate than CW − see the references in 3 above and the CIWEM Manual of Practice on Natural Wastewater Treatment.