Another thing I have noticed (generally, not Premium related) is that the playback quality option from settings has gone, the default playback quality is auto and many times even though a 4K stream is available, the auto setting uses 360p or something low like that. I am on a 1Gig connection so there's no reason it should be this way and is a behaviour I've noticed youtube doing probably to reduce bandwidth consumption server side so if you want higher quality, you have to manually select it.
Example:
On this video: