Yes, they are better alternatives. If properly built, maintained and operated, constructed wetlands can effectively remove many pollutants associated with municipal and industrial wastewater and storm water. Such systems are especially efficient at removing contaminants such as BOD, suspended solids, nitrogen, phosphorus, hydrocarbons, and even metals. They are cost efficient in terms of construction, operations and maintenance.
constructed wetlands are a great way for small-scale wastewater treatment, say up to a few hundred people.
But if you want to treat the results of thousands or more (say a town or a city) it will be unpractical.
Constructed wetlands take space and time and are not necessarily cheap to build, it depends on the location, land cost, slopes etc. They are also slow compared to activated sludge facilities.
They can be used as alternatives only for small-scale wastewater treatment plants. They are efficient in removing pollutants only if it is operated and maintained properly.
Constructed wetlands take up a huge space and leave a big footprint. Also, it is a climate-dependence process, so they are not efficient in cold and polar regions.