Development will surely follow. They are expensive to build and maintain. Clackamas County understands the lesson. None lies within an urban growth boundary. The systems experience operational and maintenance problems and failure.
Many private systems do fail.
The Donald interchange on Interstate 5, midway between Portland and Salem, lies outside any urban growth boundary. Three of the four quadrants of the interchange are developed in highway commercial uses. Each has been served by a small, private wastewater system. The small size of the systems has limited the amount of development that could happen at the interchange. To rescue the failed systems, the city of Donald is extending its sewer line outside its urban growth boundary.