From my experience, scrollable tab bars can be nice – but that is if the user actually knows that the bar can be scrolled. For example, in VSCOcam (image editing app) I discovered that the tab bar can be swiped/scrolled after two months of using the app, by accident. After showing this to a few other people, I can say that most of them didn't know that either. The fact that the part of the tab that was displayed by default already seemed to have enough options/tools also played a role, I guess.

And in the Twitter, they have this thing that from the timeline you can swipe to "Discover", and it's only shown by a small set of "pagination" circles on top – there's no explicit mention of this, and the circles are kind of easy to miss.