I used UIKit and Bootstrap many times. I love Bootstrap for fast, easy layouts to web apps. Clean code, easiest and fastest way to write layouts for front end if you learned something before about Bootstrap. Now in React I use React-Boostrap too. About UIKit I can say its nice idea. It's easier than Bootstrap. This is good option for trainee developer to learn how u should create layout of your website, but for me UIKit have not enough functions. If you need to create something complicated, u have an error in your mind. You must create amazing code combinations for UIKit where in Bootstrap in the same ideas you have easy solutions.