Obviously, although SharePoint Designer 2010 is the ideal tool for customizing, designing, integrating additional functionality, and prototyping SharePoint sites, it does have several limitations. The key limitations are mentioned here.
SharePoint Designer is not Visual Studio, so it cannot be used in the following scenarios:
• As a compiler, that is, code compilation; it does not include a code behind (page) option and cannot attach .NET or SharePoint classes.
• As a tool for creating complex Web parts, Features, or Solutions, such as packages for deploying common customizations to SharePoint servers or across multiple SharePoint site collections.
• For the creation of workflow activities.
• As a debugger. ...