Roku doesn't push channel updates. This is the sole responsibility of the channel owner/developer. So Google pushed it. Yes, it comes from the Roku update servers, but Roku has nothing to do with the app/channel (with a very few exceptions, like the Roku Media Player and Roku Channel).
While I'm currently using my 4, and not the Ultra, I am having no issues with YouTube. I've watched 1080, 720, and SD material.