The major drawback of Platform as a Service is that it may lock you in to the use of a particular development environment and stack of software components

Explanation

Platform as a Service offerings usually have some proprietary elements (perhaps the development tools or even component libraries).  Consequently, you may be wedded to the vendor’s platform and unable to move your applications elsewhere without rewriting them to some degree.  If you suddenly become dissatisfied with your Platform as a Service provider, you may face very high expenses when you suddenly need to rewrite the applications to satisfy the requirements of another PaaS vendor