[zfran]

I'd say back-end. The front-end of any software no matter how complex it is it will always run on a single device, while a scalable back-end could imply a complex architecture composed of multiple components running across multiple servers, all working together to serve the front-end.

[proexploit]

A single device that can be any out of 100's of devices with different browsers, screen sizes, densities and capabilities...

[sehr]

Alternatively, the front end can be run on hundreds of different devices and differing environments, while the back end's environment is completely and utterly locked down.

YMMV