Not sure if this has been addressed elsewhere but with the new update I can no longer choose what quality to watch a given video at so I can't force the video I'm watching to be displayed at the highest resolution. I have a 4K TV and the highest any vid will play at is 1080p.
The hate for the new YouTube 'leanback' is a hot topic. Uninstall the update, turn off autoupdates, and all will be well.