Limited knowledge: We didn't write that code, so we won't always be able to understand what we're doing and why the stuff we're using behaves like that
Performance heavy: Even the most lightweight and modular framework will undoubtedly be packed with a lot of stuff we won't be using in our project; these contents will be sent by the web server and loaded by the client anyway
Updating issues: Whenever an improved build of the framework is out, we will have to choose between updating it and taking the risk of breaking something, and not updating it and risking losing the bug fixes and the added/improved support for the new CSS standards