Can WordPress replace server-side frameworks (like Django and Ruby on Rails)?
While WordPress is flexible it runs a lot slower on a larger scale. Because of it’s foundation you get a pretty flexible runtime modification of behavior, which can open up a can of worms on your system. Simply put, it can start all sorts of extraneous processes that clutter your server and the browser.
Once you open the door to changing WordPress to do more than it’s intended (as a CMS) you start running against the way the code was originally designed to run. Working outside of those bounds can add tons and tons of extra code what would not normally be needed.
That being said, some frontend changes work great when they are designed using the WordPress framework. I’d say try it and see how it goes. Some times, if it’s simple enough, it can work perfectly and there’s not much need to scale, for instance. However, if you need this frontend build that scales while maintaining WordPress’s efficiency, you will probably run into road blocks, which you could avoid using serverside frameworks.