In my opinion Bootstrap is not a good choice for complex websites:
- Bootstrap has many unused classes and elements
- All form elements have bootstrap design (sometimes you don't really need it)
- I saw many web designs (responsive & adaptive) and can not remember one of them that use 100% of bootstrap features....so, maybe you don't need it at all?
- In my projects, I used more clear and lightweight frameworks (like KUBE or RESPONSIVA)