You can install npm libraries. You can connect to DBMS servers (not sure about arbitrary web servers). I believe they offer iframes to facilitate embedding but not frontend components.
Interactive vis is not very well integrated with the frontend ecosystem IMHO. Integrating a very interactive D3 vis with React, for example, can be painful.
Can you install libraries (from npm?)
Can you build and share custom components, not just notebooks?
Can you connect to a web server?