Any apparent security concern or downside of a browser-based client?

I am tasked with a web application project involving a lot of dynamic design.

I am going to build a RESTful API with Node.js with token-based authentication, and initially I thought about building another Node.js application for web-based UI, but now that I have a basic design of the API, I was wondering if it is feasible to implement all of the UI logic with JavaScript on the browser?

It would involve a HTML page, which has JavaScript that will GET/POST data from the API, and update the DOM accordingly, furthermore, I would save authentication token in cookie, and the JavaScript in browser would do everything ranging from login to updating/deleting/creating all kinds of data through the RESTful API.

I haven't heard anything quite like this, is there any security concerns? Off the top of my head, the API server would get tremendous amount of requests if attacked.