JS web app with PHP/JS Webservice

We want to build a scalable web-app and we have some questions regarding the overall design of the app.

We are thinking about build a pure JS web-app for the client and use a web-service as our php back-end.

This way, the code will be much cleaner, easy to test, well modularized, easy to convert to mobile app for phones, etc.

We want to use AngularJS for the front end and not sure what for the backend. (PHP - Yii, JS -NodeJS, etc. ?)

My only concern is the scalability/performance of such an approach. Our web app doesn't need the back-end but if the client wants to login, he will be able to retrieve more data through the web-service.

Has anybody build and app in such a way and is there any main drawback?

Thank you