In my experience, sometimes you "click" with a certain architecture, because it more or less maps to the way you work, and sometimes you don't. When you don't, it will be easier to adapt it if it isn't too opinionated.
A good example is going to be the upgrade process from 2.x to 3.x.
If you have been developing your site with Bootstrap to-date, then you've very likely been using their style classes throughout. For example, if you setup a grid on a page, perhaps you did something like this:
But with Bootstrap 3.x you'll need to change every instance of span* with col-span-*. Wouldn't it be nice if, instead of changing all of your templates, you could simply modify the LESS/CSS files using mixins?
Upgrading is just one example, another example is attempting to use Bootstrap to style an existing application where you do not have the ability (or only a limited ability) to change the HTML, but only the ability to inject a CSS "theme".
Bootstrap was so painful the moment you deviate from their styles. Similar to Rails when deviating from the "rails way".