I believe this question targets new WCF Web API and because of that it is not the same as question linked as duplicate.
WCF Web API removes some limitations of current WCF Rest support but the main disadvantage of the Web API is that it is only a draft - currently preview 4. It has no support and probably it is not supposed to be used in production environment. Also any new version can introduce breaking changes or remove whole set of features available in previous version. Because of that you should use ASP.NET MVC to build lightweight JSON service layer.
