This is a general question.
I've seen so many projects where they add the Angular 4 app or Aurelia app or any other front-end framework in the same project of the asp.net web API project.

What are the pros and cons of doing that?
If I made it this way customized only for one client. what if I wanted to have a mobile application consume the same restful web API?

Plus it's really hard to integrate angular 4 with asp.net

Note: I'm speaking of ASP.net 4X, not ASP.net Core.

Thanks very much