A fresh and engaging UI with groundbreaking ideas and techniques could possibly win you lots of fans but for the vast majority of applications (especially lob or ent) this does not make sense.
If a person isn't able to very easily customize Bootstrap with a minimal amount of CSS, I wonder if they're able to competently create their own front-end framework that works perfectly in every browser on every platform.
As someone who navigates the web primarily with a keyboard, sites that use Bootstrap tend to be more usable than sites that use front-end frameworks that were built from scratch. For example, in my experience, most designers and developers don't use :focus selectors when using :hover selectors because they only think about mouse-based navigation.
I evaluated Picnic CSS just now, and it doesn't use :focus selectors. This is what I'm talking about.
In my experience, usually when developers release a solution (e.g., a framework) that's advertised as a lightweight alternative to Solution X, the lightweight alternative isn't really an alternative for developers who understand why Solution X isn't lightweight.