Depends on the kind of product that you are trying to make, and the amount of time your potential users can wait.

While UI libraries have their own shortcomings that you have highlighted. They also give you a some foundation to start from, and not necessarily start from scratch.

But if you were to start from scratch, You'd be able to get features, tailor-made to your requirements (which might be absent in UI libraries).

The mechanics of how UI components work mostly stays the same and doesn't deprecate, though.

Ultimately, the call is all yours.

If you have a specific design that you have to abide by, you may find that trying to fit existing UI libraries to that design will be not the easiest task in the world, and it might be easier to start with custom UI components altogether.

If you are just trying to throw around a simple app as quickly as possible, UI libraries can definitely help.

Same as with Twitter Bootstrap on the web. You can use it when prototyping, but if you want something more custom, you will have to style (at least partially) the UI by yourself.

Personally, I'm not a big fan of UI libraries and implement visual components from scratch on my client projects.