They were a good solution for around 40 years from inception.
I worked for 2: Glenrothes in Fife 1976-80 & Skelmersdale 1981-84.
Glenrothes revived the central Fife region from a mining slump & was fairly successful, although rushed house-building & latent issues problematic throughout New Towns.
Skelmersdale was a disaster - merely exported Liverpool issues to an isolated neck of the woods & amplified housing issues.
Not sure the concept is right for the current decade/century.