The only problem with the YouTube App on iOS is that weird vertical video Angle. Every time you view a video on YouTube, you get black bars on the side. But this issue is now fixed. If you view a video on YouTube that has such orientation, then the screen will be filled in mobile view mode providing you the full-screen viewing experience.
The update also shows you when you are watching a vertical video by making the half screen view slightly bigger than other videos. This feature was available on Android but now its there on iOS too.